how old do you have to be?

Age doesn't matter. But, if you have kids with disabilities then the staff are so unprofessional. They treat those kids harshly

I don't believe there is an age limit, but you could always have your parents sign up for a family plan

For what? I beleive you can get a membership at any age, but there are age restrictions for different parts of the facility. By age 14 or 16 you can pretty much do anything but you need to be able to pay for the membership which most teens probably can't.