iuvenis

Iuvenis, iuvenis, m. and f., third declension. It has the same form in nominative and genitive singular, so context does the work: iuvenis venit is "the young man comes", nomen iuvenis is "the young man's name". The genitive plural is iuvenum, not iuvenium, which is the corner most learners get wrong.

The word is also an adjective, "young", with the irregular comparative iunior. The trap is the age. A Roman iuvenis was a man of military age, roughly twenty to forty-five, not a teenager, so "youth" in the English sense of a gawky adolescent is usually wrong. English junior, juvenile and rejuvenate all descend from it, as do French jeune and Spanish joven.