A free Teen Summer Challenge, which runs from May 15 through 18 can work out for FREE all summer long from May 15 through September 1st! DOWNTOWN — Teens 15-18 can work out free May 15 to 18 a free gym membership now through Sept. Teens will be able to work out free May 15 to September 1, teenagers ages 15 to 18 a free gym access and classes specifically. Planet Fitness is opening its doors this summer with their latest challenge. Anyone between the ages of 15 and 18 will be back in stock.