Good question but I don’t think you can attach an age to that. It all depends on one’s level of experience with training. I’d be more comfortable with a 15 year old kid who’s been training for 2 years while well-supervised versus a 40 year old novice on their own. But to answer your question I’ll go with 15 – but check with the gym. I started training my daughter at age 12 & she just turned 14 recently. I’m still not comfortable with just dropping her off at the gym – probably next year! There’s no science to that answer Pam – just my gut feeling as a dad.