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Longevity serum was a kind of treatment developed by Federation scientists in the 23rd century, and was used by people to assist them in remaining fit and healthier as they grew older.
By the late 2270s, Admiral James T. Kirk was using the serum which, in addition to constant exercise and a good diet, kept him in good shape. (TOS - Star Trek II Short Stories short story: "The Blaze of Glory")