I think the Bettas usually around 6 months at the shops. Because the stores want the Bettas to have pretty long fins so they will be bought. Even though the other day I saw a very small crowntail. Maybe he was a runt but he looked just like a baby Flipper.
Sorry to hear about your GF's Betta, sounds like he could not get enough air. Try to keep decor that can tangle easily out of a Betta aquarium, to prevent another loss.