Young is an adjective (young children) and a noun (the young).
The word young is an adjective. It means in the early part of life.
A complete sentence.
Younger is an adjective (comparative of young).
adverb
Fellow is a noun (a young fellow) and an adjective (my fellow students).
The word youth is a noun. A youth is a young person.
part of speech
The part of speech for this particular word is a noun.
adverb
what part of speech is work
what part of speech is beneath
the part of speech sashay is a averb