Sure, Peter Parker technically was a teenager, not an adult, but that doesn't mean an adult shouldn't portray him in one of these licensed Spider-Man costumes. After all, have you seen the comic book covers? The muscular definition was not that of some scrawny high school nerd.

Also, teenagers know very little about responsibility, so his uncle Ben's advice would be better considered by a more mature person. When you assume the identity of Spider-Man, you can be that person. Maybe you aren't totally mature, after all you are about to dress up like a comic book character, but you can definitely be the man Peter Parker was trying to become.