If you absolutely want to turn the knife with that additional never grew up drama, I'd bump the kid up to 16. Though 16 year old boys were treated as men at that time in a lot of places, that's still a really volatile age. Additionally, I've known 16 years (by dress and demeanor) that could visibly be confused with adults (until you have a conversation with them), but I've never seen a 13 year old that I'd confuse as an adult.

Also, why do you feel the Toreador character is a boring option? Brujah and Toreador are two of the most dynamic clans who Embrace a diverse range of people, for reasons that can range from droll to really interesting.