For legal protection, nudist venues only allow those under 18 if they are accompanied by a parent or legal guardian. The place may be completely open and welcoming to attendees of all ages, but they must have the minor's adult caretaker in attendance because they simply can't risk allowing an unaccompanied minor to attend and have a resulting lawsuit from a parent. That's just how it is regardless of how perfectly safe naturist/nudist venues are.
Now, there are some resorts that are 18+ only, no minors allowed, and that's because they want to give their patrons a place to go where they can get away from the kids for a while (and also for those who just don't like kids). All other rules of conduct remain in effect exactly like any other family-friendly resort.
Private naturist groups unaffiliated with any resort or similar dedicated property most likely follow the exact same sort of protocol, requiring a parent or legal guardian to also attend. The individual could always try contacting the group about joining up, but it's highly unlikely the response would be to come on over without concern about parental permission. If they did, there again they'd be setting themselves up for legal trouble.
naturist since 1990