I read that there is pressure on boys to appear nonthreatening for as long as possible. They benefit when the growth spurt from aw cute to damn hot is quick.
I never knew (and still, don't know) to what degree this trait is actually universal. (ie, perhaps it's just a weird quirk of teen magazines in the last 30 years) Do you have links to that study or anything like it? I'd like to check it out.