youth has 7 distinct synonyms spread across 4 of its 6 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a young person (especially a young man or boy)
the time of life between childhood and maturity
early maturity; the state of being young or immature or inexperienced
an early period of development
“during the youth of the project”
the freshness and vitality characteristic of a young person
Synonyms, antonyms and the broader and narrower relations for youth come from Princeton University’s WordNet. Full sources and licences.