He was a very invigorating young man.
That young man is a member of a motorcycle gang.
Why do youths in Japan try to hide their nasty smiles when they’re up to something?
Someone who is blue in their years. This makes sense when you know that Asian babies are often born with blue butts, which you can associate with being young. So, Blue = Young (not only in this word, but in others as well). Someone who is blue in their years is a youth.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.