My source on Sarah's birthdate is probably the Sias books (which are 12 time zones away from me right now). She is not one of my ancestors and so not someone I've pursued at all. The Sias Family in America is (admittedly) a secondary source and the birthdate in my records will be amended according to your census data.
Is a gravestone a primary source? Jane's stone reads "Jane Sias / died March 9 1813 / AE 72 years", making her birthdate 1741, not 1743.
So, yes, her "biological clock" had run down by 1800 or 1805. My point about Samuel wasn't to imply that the 10 year gap was trivial. It was to say that there were children at home in 1800 and, in those circumstances, adoption was a viable (and common, especially in farm families) option and might or might not be recorded in court records.
What I find puzzling is that you pinpoint Sarah's parentage on the basis of her living "three doors down from James & Patience". These were farmers; 3 doors down could mean miles apart. John & Sarah Sias remained on the family farm in Derby and are also buried in what used to be West Derby (near Charles & Jane and James). There is a gap in AP Sias' list of their children, between Jeremiah (1804) and Cyrus (1808). I'd say that the evidence is just as strong (or just as weak) for them to be Sarah's folks as it is for James & Patience.
Ah well, unless we dredge up more evidence, I'd say that Sarah's parentage is still unsettled. I wonder what she'd say about that!
ps About Vermont. Towns and Townships meant and mean the same thing. Within the towns are villages and/or cities.