@ Shinku: Ahhhh. I guess. ^^;;
Again, the closest thing to otorashikun I can think of is otonashiku. The word otonashii means "adult-like" (mature, grown-up, etc) and comes from the word otona, which means "adult". Otonashii only becomes otonashiku when you put it directly before a verb, i.e. otonashikushite (act like an adult), otonashiku mattete (wait like an adult), otonashiku damattete (be like an adult and shut up).