Picture of Thaddeus Keeler's tombstone at findagrave. One could read it either way, 1830 or 1850. However, if you look at his wife's tombstone, also @ FAG, she died in 1836 and the 3 is all-round, no corner on the upper right. The pictures are kind of small, and the stones are weathered. At any rate, the age at death on Thaddeus's tombstone is, clearly, 72.
So: What else have we got? The Hale Index of Cemetery Inscriptions (ancestry.com $$ link) lists death as 1830, as does an Connecticut death index on ancestry.com & familysearch (no images, just index, login required), likely drawn from Hale.
There's a St. John compilation which says he was born in 1768, which would mean he died in 1840, so long as 72 is accepted as age at death. The tombstone definitely does not say died 1840. So I'm ready to discard the birth in the St. John compilation as an error.
There's a Keeler compilation (ancestry $$ link) which has him born in 1778 and died in 1850, which corresponds with how I read the tombstone.
Of relevance: I found Thaddeus in the 1850 US Census (ancestry $$ link) living with daughter Elizabeth & son-in-law William Northrop Benedict in Ridgefield, CT. Elizabeth & her husband appear in both compilations, St. John & Keeler.
Elizabeth Benedict's tombstone includes her maiden name, Keeler.
Thaddeus's father Jeremiah, in his will (ancestry.com $$ link), bequeathed the eldest son's portion to son Jeremiah. Beyond that, he makes certain provisions for son Thaddeus when he turns 14 years of age in a will dated 1780. Had Thaddeus been born in 1758, he would have been 21 or 22 at the time the will was drawn up.
Given all this, I put the dates 1778 & 1850 on the profile I created for Thaddeus, and the 1768 profile did not come up in searches. When I did identify the duplicate, despite birth year inconsistencies, I proposed a merge using 1778 & 1850. The merge was instead completed to 1758 & 1830, dates that did not appear on either profile of the proposed merge, but do correspond with FAG & Hale.
At this point, I'm hoping to enlarge the committee considering that profile. What do you think?
Bonus question: Wife Esther's last name is St. John. Or is it St John? What's the correct punctuation? They tended to use more of it back then than we do now. I don't have a definite poairion about this. Opinions from the peanut gallery?