Hi I'm nearly 13 years old, is that to young to go the gym?
If the answer is yes, could I go with a 15 year old?
If the answer is no could I go with a 15 and 26 year old?
Thanks for your help:)

Call the gym. They can tell you their rules.

Do you mean a commercial fitness gym? Generally, such gyms require an adult to sign a contract. If an adult can sign you up it will depend on the rules of the facility.

What about school? Doesn't it have a fitness program?