After I made that photo I did some more research. It measured 19.8 feet around. The oldest oak here in florida is less than an hour from my house at 24+ feet. It is estimated (they go by rings of comparable trees) near 400 yrs. The Southern Live Oaks are different from the regular oaks and redwoods of the North West.

Seen here in 2003 when I trucked it on thru there. I would guess mine was 300+ years. It is only accurate like you said if the core sample or cut it down and count the rings.

Mother nature took my Swiss Family Briggs tree from us in back to back hurricanes in 2005. Fell under it's own weights.