they are the same species as all domesticated bettas, Betta splendens. the only difference is that they're bred from lines that are physically larger than normal bettas. i've never seen any in a store, only online, but i'd imagine if someone was sending them to stores, they'd be the same age, if not younger than normal sized bettas.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)