The date of birth is inferred from her age 78 years, 7 months and 15 days at the time of her death according to the grave stone transcription records of The First Reformed Church. That record states:

"[Plot] 319 Schureman, Ann died May 25, 1800, aged 78 years, 7 months, 15 days"

She was the daughter of Jacobus Schureman (1706 – 1795) and Antje Terhune (1705 – 1766)

Thanks to Rich Billitz 51068098 for providing the following:

Schuremans, of New Jersey (1902) by Wynkoop, Richard, 1829-1913, pages 23-30 (The author is a descendant of the family):

Jacobus Schuurman married Antje Terheun (daughter of Albert Terheun) of Flatbush, L.I. in 1720 or 1721. They settled at Three Mile Run (near New Brunswick), NJ.

Jacobus' and Antje's 1st child was Ann - b: 10 Oct 1721; buried at First Dutch Church in New Brunswick, NJ. The inscription on her headstone shows 'In memory of Ann Schureman who departed this life May 25th 1800, aged 78 years, 7 momths, 15 days.' In an earlier edition of this genealogy, it was assumed (in error) that Ann's inscription belonged to Antje (De Riemer) Stryker, the wife of Ann's brother John. Eleanor Voohees, her brother Ferdinand's widow, was buried next to Ann. Ann never married. (Having 2 people named Ann Schureman in the same family who were born and died in the same years can be quite confusing.)

Their 4th child was John - b: 27 Feb 1729; m: Antje (De Riemer) Stryker in Feb 1751; d: 6 Jul 1795. Antje De Riemer was daughter of Isaac De Riemer and Antje Woertman; baptised: 4 Oct 1721 in NYC; widow of Peter Stryker; marriage bond for 2nd marriage: 22 Feb 1750 (according to legal calendar) which means 1751; d: 1800. Antje's will (as a widow of John Schureman of New Brunswick) was dated 16 Oct 1795 and proved 23 Jul 1800.

Their 5th child was Ferdinand - b: ~1731; d: before Jun 1782. He married Eleanor Voohees, who was buried next to Ann Schureman, Fedinand's oldest sister.

https://archive.org/details/schuremansofnewj00wynk/page/22/mode/2up?q=antje+1721+ferdinand