"18-year-old is not afraid of youth" may mean: 18-year-old people have set their goals in life, and young people are fearless and forge ahead.
This sentence imitates classical Chinese, but the collocation of words is improper and the grammar is unreasonable.
"Eighteen-year-old ambition" imitates "thirty years old", and "He" modifies the adverbial "eighteen years old", but "ambition" is a noun and ambition in the sentence, so it cannot be used as the predicate head word and is not modified by adverbial.
Not afraid of youth means "not afraid of youth", which is obviously far from "young people are fearless"