Free Membership at Planet Fitness For Teens Ages 15-18. Valid May 15th through September 1st at any Planet Fitness Location. To sign up, Starting on May 15th, visit any Planet Fitness location in the United States or Canada. Online signups are not available. Teens under 18 must bring a parent/guardian to sign up. Once the parent/guardian waiver is signed for teens under 18, teens can workout alone. Students who are already 18 do not need a parent or guardian to be present during the sign-up process.