How did Jason Voorhees grow up if he drowned as a child?
According to info gathered from a few of the movies, Jason has the ability to regenerate. Not fully which is why he's so messed up looking but enough to bring himself back to life. So yes, he did drown but they stated that since he watched his mother beheaded in the first film, he came back up from the lake to exact his revenge. In Jason X, the one set in space, one of the characters says all of the ways they tried to execute him but they decide to cryo freeze him since he just won't die. At the end he re-enters the atmosphere and burns up. The movies ends with only seeing his mask at the bottom of the lake and you hear the ever famous ch-ch-ch sound(forgive the spelling there) as 2 teens say "let's go check it out". Just 1 more hint that he can't be truly killed.