Valentudinary is a rarely used word that refers to someone who is in the prime of their life. It is spelled "vuh-len-too-di-nair-ee" and is pronounced /ˌvælənˈtuːdənəri/. The first syllable is pronounced with a short "a" sound, followed by a schwa sound, and then a long "oo" sound. The final syllable has a stressed "nair" sound, followed by a schwa sound and then a short "i" sound. Despite its infrequent use, Valentudinary is a sophisticated term that could be useful in certain contexts.
Valetudinarian.
A practical medical dictionary. By Stedman, Thomas Lathrop. Published 1920.