Is it normal for a 12 year old boy to have an adult sized penis?

yesterday i was at my cousin's house and we were sitting on the couch watching tv. i put my feet on his lap thinking it was no big deal and after a few seconds i started to feel something hard under his pants. i asked if it was his penis and he laughed and said yes. my feet weren't even on his crotch so i looked and saw there was a bulge in his pants that went halfway down to his knee! i told him theres no way that whole thing is his penis. he asked if i wanted to see it and i was curious so i said yes. he pulled his pants down to show it to me and it was huge! i don't know how big exactly but i wear a size 7 and it was at least as big as my foot, and i even had thick socks on! i asked if he ever measured it before and he said yes. he said the last time he measured it it was over 8 inches and don't doubt it! i know he started puberty early and hes already almost 6 feet tall but how common is it for boys to have penises that big when they're only 12?? what age do they stop growing?