@Tawnie0
Age is not in the AirBNB non-discrimination policy. At some point, you are "extending credit" to your guests, i.e., trusting them to behave with a very expensive item (your home). Also, with guests under 21, liquor liability and other issues come into play. I've never heard of anyone questioning minimum age to rent a car, for example. And obviously the "big high-school party" or "big college party" is exactly what hosts (and our neighbors, and our local lawmakers) don't want to encourage in our rentals.
Re-read the nondiscrimination policy--I think you are okay saying "no" to teenagers.