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.
Younger is an adjective (comparative of young).
The phrase "I was then too young" is a complete sentence comprised of a subject ("I") and a verb ("was"), therefore it is a declarative sentence. It does not represent a single part of speech as it contains multiple elements functioning together.
adverb
The word youth is a noun. A youth is a young person.
"Little" is an adjective, describing the boy. "Boy" is a noun, referring to a young male.
The part of speech for this particular word is a noun.
part of speech
The part of speech for "answer" is a noun.
what part of speech is beneath
adverb
The part of speech for "explicit" is an adjective.