You’ll need adult supervision under 16. You can’t lie about your age, I don’t know the process as I joined PureGym when I was 18, but there is a minimum age for a reason. If anything happens to you, they could get in trouble and obviously they don’t want that. That’s why you need adult supervision, makes them less liable.
Can’t you go with someone at least 18? Otherwise I would say wait and try some home stuff etc