Do his rays split again at the end? He might be a combtail or a single ray crowntail. Also, no one can ever be sure of the age of pet store bettas, but I would guess somewhere around 4-6 months for him? Don't take my word on it though. I would ask someone who breeds bettas, as they would know the growth rate of the fry.
100g Native Fish Pond, 55g Goldfish/Catfish Community, 50g Plant Pond, 40gB Tetra/Cory Community, 20gHM Community, 20gH Turtle (Bender [RES]), 20gL Turtle (Kif [YBS]), 15g Reef, 5.5g Betta (Okami), 5gFBH Betta (Pron), 5gHex Betta (Twister)
2 dogs, 13 cats, 2 turtles, 1 bearded dragon and 1 crested gecko