Glad to see your post on Genforum.I hope we can help each other.I've been researching these Neills for almost 40 years, and I still don't have them all straight.I tell my kids they are not the ONLY reason I turned gray!
I am a descendant of William Neill (m. Mary Clinton, 1750, New Castle Co, DE), brother of the Andrew in question.
The only "proof" I have of Andrew's age is that he bought land on the Middle Fork of Lambert's Mill Creek (adjacent to his brother William) in 1756.He would have had to be at least 21 to make that purchase in his own right, as I understand it.Of course, he could have been older, and probably was.The father of Andrew and William (m Mary Clinton) is also named William.Earlier researchers had placed the father's date of birth around 1711, but my research on son William has caused me to move the father's year of birth back to no later than 1700.Father William died c1769 in Rowan County, NC.His birth date could have been as early as 1690, I suppose.I haven't been able to find the information I need to support any date of birth for him.
Andrew's will was probated 20 Nov 1814, Iredell County, NC, (Will Book I-399)
My mother pronounced her maiden name as you say you pronounce it.However, apparently the older pronunciation in Colonial times was closer to Nail.Cynthia Forgey Neill, widow of John Neill (son of my William and Mary) remarked in a widow's pension app made in Logan County, KY, that the name Neill was sometimes pronounced "Nail" in North Carolina, where they had come from.
I would not be surprised to discover that your Sarah may be placed in the wrong generation, and that she may, indeed, be a sister to Andrew rather than a daughter, especially given your information from the tombstone.
This family is the first family I worked on when I started this journey, and I trusted a lot of information given me by other researchers.Some of it is good, some not, and I am still sorting it out.
If you care to continue a discussion privately, please click on my name above, and my current email address will appear.There is a possibility that it will change in the next few months as we will be moving at that time, and I will change to the new address as soon as I get it.