Really?
The guy just wants xp. Let him in...you're pugging anyway
If you don't want to pug, don't put an lfm up. But since you are pugging, a level 12 in a level 13-15 group is hardly worth the effort to decline let alone have a conversation about.
If I was 12 and saw your group up with nothing else going on, I'd certainly request to run with you. And you'd be better off for letting me in, for letting Clay in or any number of other people in this thread.
Broaden your horizons...you might actually learn something.