Many fitness centers have age minimums, mainly to keep kids safe. Equipment like stationary bikes are often designed for people around 5 feet or taller, and treadmills can pose a real injury risk to children. But some family-friendly gyms welcome children to work out alongside a parent. Kids as young as 7 or 8 can begin resistance training using weights. They should be supervised by a certified strength-training expert to ensure they use proper techniques to prevent injuries.