I feel SmartFlex "Maintenance" to my 7 year old (who has no existing joint problems). I'm not sure if you consider that "young". It's formulated for horses without existing joint problems, but that are working in a way that makes them "at risk" for joint problems. This is the spiel on the smartflex packaging:
SmartFlex I provides basic support of equine joint health. SmartFlex I helps maintain healthy joint function by ensuring availability of the biochemical building blocks used by the horse’s body during the dynamic process of tissue breakdown and repair that occurs with training.I really like how that bolded part sounds. Maybe it's just marketing but eh, if there's a chance that for $16 a month I can keep my horse comfortable and able to do his job for a few years later in life, it seems worth it.
SmartFlex I is a concentrated formulation consisting of 4,000mg of Glucosamine, 5,000mg of MSM, and 1,250mg of Vitamin C