An age range tag of sorts might be better. For example 'Teen' 'Young Adult' 'Adult'. Then again, we might have the problem of what the 'Adult' tag can mean. Still, an age range tag could also be used for giving a general sense of the age of the characters in the RP.

I do agree though that it has lost its meaning - or, rather, its meaning is a bit of a grey area. The official description of '18+' is 'Involves adult themes'. Depending on how you interpret that, it's the difference between an 'R' rated movie and a 'XXX' one.

I think it's an interesting issue, and love to see some more discussion on it.