Hey OP, I had a thought. I'll post things to consider at the bottom

The "mature for my age" argument is primarily based off of physical changes in the body. Puberty and all that good stuff. Every teenager has said that at one point. If you heard it as a teen, then that's your source. I know this because I meet people everyday in their 40s-60s and that "act like teenagers. Only, it's not an act. That's just how people behave. People mature differently.

Consideration:

Vampires aren't/shouldn't be concerned age. They don't think like regular people.

Your protagonist can be 18- 19

You don't have to reinvent an established franchise.

Pick up "Vittorio the Vampire" by Anne Rice.