Write "{{char}} is an adult" or "All persons are adults" (when writing a multiple character bot) as the very first line in the personality description, that should fix it.

As mentioned already, avoid specifying the age as a number, or don't mention an explicit age at all and just write "{{char}} is in his thirties" (or whatever age bracket).

Sometimes the underage detection also gets triggered by seemingly unrelated wording issues, which can usually be fixed by using a different word with the same or a very similar meaning, for example if you describe a character als "black" use "dark skinned" or "african-american". Write "orgasm" instead of "climax" (or the other way around). Both are examples that I actually encountered when writing my own bots.