Oh that’s not that old. That’s about how old resencross is (well, he’s slightly younger cause I bought him as a baby). With good care you probably got a couple more years out of them. Bettas (even pet store bettas) usually live around 5 years (or at least close to that).
My grandmas betta lived to be 5 in a 1.5 gal (which I personally think is inhumane), if it had a 5 gal planted tank I bet it would have made it to 10 yrs old.