PETERS, C.J.

This case concerns the validity of a stipulated judgment modifying the provision of a dissolution judgment that had previously obligated one of the parties to pay postmajority support for the parties' children. The plaintiff, Nancy C. Hirtle, filed a four count motion asking that the defendant, Robert L. Hirtle, Jr., be held in contempt for failing to pay child support as well as various other financial obligations stipulated in their modified dissolution judgment. The trial court, Steinberg, J., without making a finding of contempt, ordered the defendant to pay $2300 for support calculated at the rate set in the modified judgment. After a rehearing at the defendant's request, the court limited the support arrearage to the lesser rate set in the original judgment. The plaintiff appealed this reduction in the support order to the Appellate Court. We transferred her appeal here in accordance with the provisions of Practice Book 4023 and now reverse.

The underlying facts are established by the record. In 1980, the trial court, Cohen, J., rendered a judgment dissolving the marriage of the parties. The decree of dissolution incorporated a written separation agreement between the parties specifying that the defendant would "pay to the plaintiff as child support, the sum of $35.00 per week per child, a total of $70.00 per week commencing February 15, 1980. If either child is enrolled as a full time college student in a baccalaureate program, [the] defendant shall continue to pay the aforesaid support to [the] plaintiff for such child for each full week of actual residence by said child with [the] plaintiff." While paying support for the two children, the defendant had "the exclusive right to claim each child as an exemption for federal and state income tax purposes." The decree of dissolution also obligated the defendant to pay the plaintiff alimony in the amount of $150 per week, until the defendant reached the age of sixty-five or until the plaintiff remarried or died.

A stipulated judgment rendered on September 18, 1984, by the court, F. X. Hennessy, J., modified, in part, the terms of the 1980 decree of dissolution. Because of the plaintiff's remarriage, her alimony was terminated. The modified decree of dissolution further provided that: "The support order for the child Robin is modified so that the new support order is $115 for her support weekly." Robin, the parties' younger daughter, was then still a minor. No appeal was taken from this stipulated judgment.

The defendant stopped paying child support for Robin when she became eighteen. The plaintiff then brought the present proceedings asking that the defendant be held in contempt for his failure to honor his financial obligations under the modified decree of dissolution. In calculating the amount of child support to which the plaintiff was entitled, the trial court, Steinberg, J., first awarded her $115 per week, in accordance with the modified decree of dissolution. After reargument, however, the court limited her recovery to $35 per week in accordance with the terms of the original, unmodified decree of dissolution. The trial court based its revised conclusion on two grounds. It determined that no court had the authority to modify postmajority support without an express written agreement of the parties and that the September 18, 1984 stipulated judgment did not manifest a written agreement of the parties for postmajority support.

The plaintiff's appeal raises three issues. She maintains that the trial court: (1) exceeded its authority in granting the defendant's motion to open and reargue; (2) mistakenly concluded that it lacked independent authority to modify the defendant's obligation to provide postmajority support for his daughter; and (3) mistakenly found that the September, 1984 modification did not incorporate an agreement by the defendant to pay postmajority support to his daughter. We agree with the trial court's resolution of the first two issues, but not with its disposition of the third.

I

The plaintiff maintains that the trial court, having rendered a final judgment on November 27, 1989, that the defendant owed a child support arrearage in the amount of $115 per week, lacked the authority thereafter to grant the defendant's motion, dated December 12, 1989, to open the judgment and to reargue. The plaintiff contends that a valid motion to open a judgment and to reargue requires at least an allegation that the moving party is relying upon newly discovered evidence not previously discovered or discoverable. We disagree.

The principles that govern motions to open or set aside a civil judgment are well established. Within four months of the date of the original judgment, Practice Book 326 vests discretion in the trial court to determine whether there is a good and compelling reason for its modification or vacation. TLC Development, Inc. v. Planning Zoning Commission, 215 Conn. 527, 533-34, 577 A.2d 288 (1990); Gillis v. Gillis, 214 Conn. 336, 340-41, 572 A.2d 323 (1990); Sanchez v. Warden, 214 Conn. 23, 35, 570 A.2d 673 (1990); Acheson v. White, 195 Conn. 211, 214-15, 487 A.2d 197 (1985); Steve Viglione Sheet Metal Co. v. Sakonchick, 190 Conn. 707, 713, 462 A.2d 1037 (1983). The present record does not demonstrate that the trial court acted unreasonably or in clear abuse of its discretion.

II

The plaintiff next maintains that the trial court misconstrued the statutory and case law authority governing support agreements for children past the age of majority. Contrary to the ruling of the trial court, the plaintiff contends that a court has jurisdiction to modify an agreement for postmajority support even where the parties have not agreed in writing to such a modification. We agree with the trial court's holding that General Statutes 46b-66 requires agreements for the modification of postmajority support to be in writing.

The 1977 amendment to 46b-66 was a legislative response to our holding, in Sillman v. Sillman, 168 Conn. 144, 150, 358 A.2d 150 (1975); see also Kennedy v. Kennedy, 177 Conn. 47, 49-50, 411 A.2d 25 (1979); Simon v. Simon, 170 Conn. 24, 363 A.2d 1054 (1975); that the Superior Court's jurisdiction to make orders of child support extended only to minor children. "The purpose of the amendment was to ensure that a written agreement involving support for adult children and submitted to the court by the parties in connection with an action for a dissolution of marriage could be incorporated into the judgment or orders of the court, thereby making such orders enforceable through contempt proceedings." Arseniadis v. Arseniadis, 2 Conn. App. 239, 244, 477 A.2d 152 (1984); Albrecht v. Albrecht, 19 Conn. App. 146, 154, 562 A.2d 528, cert. denied, 212 Conn. 813, 565 A.2d 534 (1989).

Although General Statutes 46b-86 (a) confers authority upon the Superior Court to modify "any final order" for the payment of support, that authority must be read in conjunction with the jurisdictional limitations imposed by 46b-66. As the Appellate Court held in Albrecht v. Albrecht, supra, 155-57, the same jurisdictional rules apply to the incorporation of an order for postmajority support in the original dissolution proceedings as to a subsequent order modifying postmajority support. In both cases, 46b-66 requires a written agreement of the parties.

The decision of this court in Barnard v. Barnard, 214 Conn. 99, 115, 570 A.2d 690 (1990), does not compel a different conclusion. In that case, the issue was whether, as 46b-86 (a) expressly permits, the parties had effectively precluded any subsequent modification of postmajority support by the terms of the separation agreement that had been incorporated into the decree dissolving their marriage. In that context, we noted that the postmajority support order could be judicially modified "only if the agreement so incorporated does not preclude modification." Id., 114. Our repeated citations of Albrecht v. Albrecht, supra, indicate that we did not intend to foreclose the consideration of other constraints on the validity of a modification of a post-majority support order. Barnard v. Barnard, supra, 113-14. We therefore concur with the Appellate Court in Albrecht v. Albrecht, supra, and with the trial court in this case, that a written agreement is a jurisdictional prerequisite to the valid modification of an order for postmajority support.

III

The plaintiff finally challenges the finding of the trial court that the parties had not agreed to a written modification of the defendant's obligation to provide postmajority support for his younger daughter. We agree with the plaintiff that this finding cannot be sustained.

In addressing this question, we need to recall the procedural context in which it is raised. In 1984, the defendant, himself an attorney, agreed through counsel to a stipulated judgment modifying the amount of his obligation to pay postmajority support. He took no appeal from that stipulated judgment, and even now does not allege that it was obtained by fraud, duress, accident or mistake. Connecticut Pharmaceutical Assn. v. Milano, 191 Conn. 555, 558, 468 A.2d 1230 (1983); Celanese Fiber v. Pic Yarns, Inc., 184 Conn. 461, 466, 440 A.2d 159 (1981); Kenworthy v. Kenworthy, 180 Conn. 129, 131, 429 A.2d 837 (1980). He has never filed a motion to modify its terms. Instead, he now resists its enforcement by a collateral attack on its validity. Final judgments are, however, presumptively valid; Lampson Lumber Co. v. Hoer, 139 Conn. 294, 298, 93 A.2d 143 (1952); Coit v. Haven, 30 Conn. 190, 197-98 (1861); Robertson v. Fazzalaro, 33 Conn. Sup. 569, 573-74, 363 A.2d 755 (1976); and collateral attacks on their validity are disfavored. Santora v. Miklus, 199 Conn. 179, 188, 506 A.2d 549 (1986); Monroe v. Monroe, 177 Conn. 173, 177-78, 413 A.2d 819 (1979), appeal dismissed, 444 U.S. 801, 100 S.Ct. 20, 62 L.Ed.2d 14 (1979); 1 Restatement (Second), Judgments (1982) 12.

The defendant's collateral attack can be sustained only if the record demonstrates that the trial court, F. X. Hennessy, J., had no subject matter jurisdiction to increase the defendant's support obligation because the court was never presented a written agreement of modification. Since "[j]urisdictional facts are conclusively presumed in domestic courts of general jurisdiction, even when not found by the court, unless the record itself shows the contrary"; Robertson v. Fazzalaro, supra, 573; it was the defendant's burden to prove the existence of a jurisdictional deficiency. Cf. Packer Plastics, Inc. v. Laundon, 214 Conn. 52, 57, 570 A.2d 687 (1990). Accordingly, the defendant cannot prevail if there is an absence of evidence to establish whether the stipulated judgment modifying the terms of the dissolution decree was grounded in the written agreement of the parties.

The evidence proffered to show a lack of jurisdiction was the record of the proceedings before Judge Hennessy. Although the defendant testified, in the proceedings before Judge Steinberg, that he had participated in oral discussions antecedent to the modified judgment, he acknowledged that he had not been present in court at the time of the stipulated judgment. He therefore had no personal knowledge of what had transpired. The court record consists of the concededly skeletonic description of the terms of the modified judgment, initialed by M.H. and signed "Hennessy, J.," and the transcript of the proceedings before Judge Hennessy. The transcript reveals the following. After verifying the presence of counsel for the parties, the court stated: "[T]he agreement has been given to me by the parties as to the judgment of April 9th, 1980, [which] is modified as follows: Alimony award entered therein is terminated because the plaintiff has remarried. The support order for the child Robin is modified so that the new support order is one hundred and fifteen dollars for her support weekly. Is that the agreement, gentlemen?" Upon obtaining the assent of counsel, the court then stated: "It is so ordered." Notably, the court, relying upon "the agreement . . . given to me," itself stated the terms of the modification, rather than inviting an oral recital by counsel.

On this meager record, the defendant has failed to overcome the presumption that Judge Hennessy was reading from a written document when he described the terms of the modified support order to which the parties had agreed. The modified judgment of postmajority support has therefore not been shown to be jurisdictionally defective. Further proceedings will be required to determine the amount of the defendant's arrearage in accordance with the terms of the modified judgment.