You are mixing syntactic functions (how words and phrases relate to each other within the context of a sentence) with parts of speech or word categories (how words are classified on an individual basis).
"He is fifty-four years old" is a sentence formed by:
- Subject: "He"
- Predicate: "is fifty-four years old"
Within the predicate, we have:
- Linking or copulative verb: "is"
- Subject complement: "fifty-four years old"
And here comes the most challenging part: deciding how the subject complement is structured and works.
To me, a feasible parsing would be saying that "old" is the head of the subject complement and that the noun phrase "fifty-four years" premodifies "old". Actually, that sentence can be the answer to the question:
in which "how" (just as "fifty-four years" in the answer) intensifies or modifies "old".