You might run a sim or parcel that is quite mild in content but still contains enough adult material to make you feel compelled to ban 16 and 17-year-olds off it. There’s an easy fix for this in your land’s access tab:
The ban lines will appear only for people who are 17 or younger, which is practically nobody.
Parcels like these might include erotic art galleries, discussion groups in adult topics, dating venues, and parcels with no obviously sexual content but containing adult animations in the furniture.