Within my District, Local Government Authority controlled Gyms will only allow boys and girls aged 16 or over to use the equipment .....
..... and even then,
only after they have completed a supervised "induction session" to demonstrate & explain how to use the equipment, in accordance with the relevant "Health and Safety" requirements and legislation.
I should however point out that this policy is not mandatory, and there is in principle nothing to stop an independent Gym making its equipment available for younger people to use.
. I have no doubt that most schools have Gymnasia with a range of equipment like that to which you allude, and that they make it available for their pupils to use under appropriate supervision. . I should however point out that I am of the opinion that teenagers should be kept out of such places while normal people are trying to use the equipment.
Best wishes - Majikthise. .
I really think it depends on where you live cause almost every guy I know works out at gyms & they range from ages 13-19, same with some girls too.