Turk, my first lived for almost 4 years with me and he was fully grown when we got him so I think he was about 5. He was the last betta sold by my Wal-Mart. I bought him as a decoration for my wedding, assuming someone would want him. No one did, so I took him home and fell in love with him. We called him Methuselah as he got really old.
Second betta, Azul, he was full grown at purchase and lived about 2 years.
Both of these, I had no idea how to take proper care. They lived in a small 1/2 - 3/4 gallon squared candy-shoppe style vase with a sterling silver lid. No heater, Fed exclusively hikari pellets. I changed the water once a week, 100%, using only aged tap water. I cleaned out the marbles, the non-smooth fake coral decor, and the vase with dawn soap and hot water. They never had clamped fins or visible illness, but their bodies, in retrospect was dull and paler than I know they should have been and they were certainly lethargic. We never knew how active a properly kept betta would be!
I'm glad that despite our terrible care, they lived a while, but we obviously do not condone that type of "care" and would treat another in such way. But petsmart told us the wrong info- you should be able to trust a pet store on how to care for commonly sold pet such as a betta.
Our current betta, I estimate he was not quite full grown at purchase, as he has grown visibly since we got him. I think he was close to a year though.