Do you mean "sexy" in terms of clothing, the character themselves, or both? Of course there's a line to be drawn when it comes to clothing (Satsuki and Ryuko in their battle armor are definitely inappropriate), but when I was 16 my first cosplay consisted of a bare midriff and a skirt with a somewhat-high slit in it. Looking back I'm not sure how my parents let me wear that. The character herself was from a hack-and-slash game rated for teens, and it was mostly kids my age who played it. She wasn't exactly Jessica Rabbit but the costume probably wasn't something that too many 16-year olds would wear to a convention. No one seemed to have a problem with it.
I feel like characters that are primarily known for being overtly sexual (like Jessica) are inappropriate for cosplayers under 18. Characters in outfits that are MEANT to be revealing should also be discouraged. But for other characters, like Catwoman, it takes a good deal of thought before making a decision.
Sailor Star Maker (Sailor Moon)/fem! Rei (Free!)/'Red' Jasmine (Disney's Aladdin)/Suzuya Juuzou (Tokyo Ghoul)
World Cosplay Summit // Tonari de Cos (Comiket) // Anime USA