How old must you be to belong to Silver sneakers?
I think you just have to be eligible for Medicare, or be a retired member of a participating plan. You can check those here: http://www.silversneakers.com/TellMeEverything/WhichhealthplansofferSilverSneakers.aspx
One has to be over 18 years old to belong to a silver sneaker. You can visit https://www.silversneakers.com/Home/Instructors/InstructorRequirements.aspx to see other requirements.