Charlie Brown was eight years old according to the comics, which started in 1950. Assuming he aged normally starting then, he would be seventy-one now. If we assumed that he didn't age during the course of the comic series though, since he was still eight when the last one was made in 2000, that would make him twenty-one. So he's one or the other, take your pick. (In this particular image, it would seem that you made him out to be the former.)