Someone who is in their young years is a youth, or young man.

青年 refers to young adults, usually in their late teens to early twenties. The word is gender-neutral in legal or official contexts, but in everyday use 青年 usually refers to young men.

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. That said, this reading can be easy to confuse with other similar words, so here's a mnemonic to help you out:

The youth are really into sabers (せい) and nendoroids (ねん). You remember nendoroids, right? The big-headed plastic figurines from Japan? As a cool, mature young adult, you've aged out of kid stuff like toy swords and stuffed animals. Now, you're all about real sabers and collectible nendoroid figurines.