I don't know if it's a widespread rule about hottubs; it's probably pretty common. But I expect it's aimed more at little kids who would want to go in without their parents and stay in for a dangerously long time. (And by the way, there where a lot of pre-teens blithely ignoring the age 16 cut-off). If you're supervising your child, I don't think it would be a big problem. For us, it was more a matter of keeping Luke's limits in check.