"18" will become a state of mind. "Adults" will ask "minors" if they're 18, but it won't mean what it used to mean. It won't mean "are you 18 solar rotations", it will mean "Are you a green light or a red light?". Many people who don't want to have sex, or who are asexual and want that to be known, will start identifying as "minors" or "under 18s". 18 is the symbol of sex havers, so in the future, anyone of any age can identify as an 18, and all the 18s will be willful sex havers, anyone who doesn't want to be a sex haver can identify in whatever other way they choose.