single word requests - What is the term for young adult male/female (aged 18 to 25)?

When writing a book and referring to a female and male aged 18–25, what term would be most appropriate? Boy and girl seem too young, whereas man and woman seem too old.

Example sentence:

A boy/man emerged from the shadows.

Edit: I think the word I am looking for is similar to 'guy' but less American and colloquial. Any thoughts?