Sentence:  Too often a beginning bodybuilder has to do his training secretly either because his parents do n't want sonny-boy to `` lift all those old barbell things '' because `` you 'll stunt your growth '' or because <WSD>childish</WSD> taunts from his schoolmates , like `` Hey , lookit Mr. America , whaddya gonna do with all those muscles ( of which he has none at the time ) '' ? 
Meaning of the word marked with <WSD> tags:  indicating a lack of maturity