But how much of that association is a social thing?
OK, well forgetting about the male face for a minute, why is shaving under the arms considered acceptable for women, but shaving the minge is associated with looking "pre-pubescent"? Nobody accuses a woman who shaves her arms or legs as trying to look like a child.
I honestly have no idea - it probably is social convention. In my head I still have that 'pre-pubescent' association for underarm and legs, it's just not so strong. Probably due to what I've seen growing up, what you're used to - you don't tend to see many shaved pubic regions unless you peruse porn mags. And even when I have watched porn/'read' porn I more bemused more than anything else.
Personally I'm not fussed about pubes either way really, but hairy underarms are a bit of a turn off.
I had an ex who would shave everything below her neck, but often leave the few strands on her chin untouched. Not that there were that many, but I still found it a bit offputting, and one of those subjects I was too scared to approach her about ... a bit like approaching someone with BO, in case they hadn't noticed or something.