Their policy states this on their website. "If you are less than 18 years old, you may not use this Site without the prior permission and supervision of your legal guardians. Planet Fitness does not accept members under the age of 13 and does not knowingly collect information from anyone under the age of 13."
She can start at 13 and they will require proof. I tried with my 12 year old who works out at home and also looks a lot older. No exceptions.
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