Because that is the perfect time in a young man's life to pilot a super robot or mech and have a coming of age story full of tears, confused sexual feelings, and maybe some hot-blooded yelling if possible.
You don't see 7 year olds piloting mechs. You know why? They aren't going through puberty. They have no angst or confused emotions.
You maye see some 20+ year olds piloting mechs but they aren't nearly interesting enough to be a main character. They are already manly. They don't whine. They go straight for the women they want without internal conflict. Their story has already ended off-screen.