My problem is this; the words “boy” and “girl” denote youth (at least to me), while “man” and “woman” denote more of an adult. While I am aware of just adding “young” before “man” or “woman”, but that feels clunky and too formal, so is there some intermediary word between the two?
Youngster is a good word you could use. It is not too familiar, and it has an affectionate ring to it. It means:
a young person, usually an older child:
- The scheme is for youngsters between the ages of ten and 16. (Cambridge)
Youth is another option, and depending on the context it can be quite poetic. It means:
a boy or a young man:
- Gangs of youths were throwing stones and bottles at the police.
As for juvenile, it is too technical, usually used in legal writing.