An eighteen yr-old is old enough to drive a car. Old enough to wed without their parents' permission. Old enouth to go to R and NC-17 movies. Old enough to smoke. Old enough to be sent to prison if they've commited a crime. Old enough to vote. Old enough to enlist in the military and fight, maybe die, in a war.
In other words, socially and legally, they are adults, not kids. Treat them like kids and they'll act accordingly; treat them like adults, and they may well surprise you with the depth of their maturity.
Personally, I don't think we should allow our homes (or specifically, our toilets) to get grimy, just because one member of the household isn't pulling their weight; that's probably not healthy.