Pet store male bettas (PetSmart, petco) generally tend to be past their breeding prime, they tend to wait longer to ship them out, as Kytkattin said above, which results in fish with longer and more devoleved finnage... So generally around 8 months to a year old.
If you go to a fish specilizing store, however, their fish tend to be younger and in fine breeding prime. I got an opaque HM pair from my LFS, I'm guessing 6-7 months...
Older fish tend to have the "old, rough" look... Their scales may no retain that 'smooth appearance", they may have bred a few times or may have got into a fight with another fish... these battle marks will show as fins rarely ever heal up perfectly... Usually there is unwanted curling of the fins and kinks where fin healed up... They tend to be larger, too.
Younger fish have a smooth, sleek appearance, usually fins will still have a black edging which signals that it s still growing... Fins show no unessary curling, smooth scales, may be more lively and alert, all in all they're like a "hot, new package" next to some of the older citizens of bettadom
Hmm, well I have a lot of fish...