I always use a disclaimer like this one...
All characters depicted in sexual activities in the following story are 18 years old and up. The words "teenager", "teen", "college girl", "teenage slut", "cute little maid", "teenage black cocksucker" and such other similar teenage-related terms refer specifically to characters aged 18 or 19 years old.
It's based on the fact that no one can argue against the words "eight-TEEN" and "nine-TEEN". It works especially well when I set a story in a long-gone era where mentions of age can simply be left out. This disclaimer is actually the result of my thinking on how to adapt a slave revolt story set in 1st century A.D. with a main character who was originally younger than 18.
This is especially important for me since I love to include teenage girls in my stories.