titillation

Barely-legal is a "shock" word of sorts.

Yes indeed, it's intended to titillate people by mildly shocking them. Most of the time when I hear the expression 'barely legal,' it means they're desirable. They're legal, but only just. Youth is beautiful, and people know it. 'Barely legal' seems like a tacit admission of that fact while avoiding stating it flat out. Heaven forbid men admit they like underage girls!