youthful 3501 - neos {neh'-os}; including the comparative neoteros {neh-o'-ter-os}; a primary word; " new " , i.e. (of persons) {youthful}, or (of things) fresh; figuratively, regenerate: -- new, young.
youthful 3512 - neoterikos {neh-o-ter'-ik-os}; from the comparative of 3501; appertaining to younger persons, i.e. juvenile: -- {youthful}. ***. neoteros. See 3501.
youthfulness 3503 - neotes {neh-ot'-ace}; from 3501; newness, i.e. {youthfulness}: -- youth.