Erasmus Withers Allen b. Feb. 6 1711/12 in Essex County, VA married Jan. 6, 1737/38 Sarah Streshley b. Feb. 13 1715/16 in St. Anne's Parish, Essex County, VA
They moved to Spotsylvania County in the early 1740's, apparently with a number of his other cousins: John Waggener, James Herbert Waggener, and Samuel Waggener, Jr.He left Spotsylvania County about 1770, moving to Rowan County, where he joined his cousin Samuel Waggener, Jr.
They had 10 documented children (In order of birth): Mary, William, Rachael, Hannah, Amelia, Susannah, Elizabeth, Streshley, Thomas and Sarah.
Streshley Allen was born June 03, 1754.I have no other information on him.Thomas may be the Thomas Allen who married Margery Bryan.However, William Allen (Thomas' brother) had a son named Thomas who is also in the right age range.Unfortunately, the marriage record doesn't detail the father.I tend to believe the former as the family lived next door while William remained in VA.
Erasmus had a number of brothers: William, Thomas, James, Zachariah, and Andrew.I believe all the brothers are older than Erasmus, being born between 1698 and 1710.
Since Erasmus Withers Allen married Sarah Streshley, I'd guess that Streshley wouldn't be a given name in any of his brothers or cousins families (unless someone there married a Streshley, too...double marriages being relatively common in early VA).
Thus, the probability is that the Streshley Allen, b. abt 1785 is a grandson of Erasmus Withers Allen.Whether he's a son of Streshley or Thomas, I don't know.
However, I'd think that the first place to start looking is in the locations that Erasmus Withers Allen lived.Certainly, any marriage prior to moving to KY is recorded.
I should note that the early members of this family are frequently listed in records as Allin.This may have been a family spelling which later changed or just spelling by some court clerk.
Otherwise, maybe you can use DNA testing to determine if your Streshley is part of this family.While it won't determine relationships, it will show if some relationshipexists.
Hope some of this helps.Email me if you want a file with what I have on Erasmus and his ancestors.