If, and it's a genuine if, Julius
means "downy-bearded", then - male shaving habits being what they are - it should refer to the first whiskers a young gentleman sprouts. So there's your "youthful" idea.
I think we're actually up against the same mindset one sees with names like Charlotte
, where instead of thinking of them as the feminine forms of Charles
, meaning "male" or "manly", people prefer to gender-switch the definition and say that Charlotte
etc "really" mean "womanly" or "female".