There isn't a way do do that via any of the site's settings.
Any person over 18 can have an Airbnb account and make a reservation. It's against Airbnb's policy to discriminate on age.
If you can prove that your community, or building owners have rules regarding age then you may be able to persuade Airbnb to cancel any reservations from persons below the approved age.
I just had someone who is 18 book a stay at my airbnb but my listing clearly states that at least one guest must be 21 years of age "the responsible adult". What do I do?