Most of the gyms I have done consulting work for set the age at 16 for being unsupervised. 12 to 15 can come in if supervised. And under 12 not allowed in the exercise area. I advise the gyms to have a course for the 12 to 15 age group that must be passed to use the facilities with supervision. Things like gym etiquette, safety, how to use machines, free weights, etc.