In the sentence 'the boy is ten years old,' I can't understand why years is a noun. I can see ten years old as modifying boy, so it's an adjective. Individually, ten modifies years and years modifies old. So is years a noun or an adverb since it modifies the adjective old? Also what is the part of speech for ten?

  • Are you clear about this now?
    – BillJ
    Nov 30, 2023 at 17:16

2 Answers 2


The boy is ten years old.

"Ten years old" is an adjective phrase headed by the adjective "old" which is modified by the noun phrase "ten years". The latter consists of the determinative "ten" + the noun "years".

Note that the adjective phrase does not modify "boy", but is a subjective predicative complement of "is".


"years" is a special type of noun called a "unit". Units are combined with numerals to describe the amount of something. Other units are "gram", "metre" and "ounce".

You know that a "unit" like "year" is a noun because of its grammar. A noun (typically) has singular and plural forms (year/years). An adjective typically has comparative forms (eg old/older). But you can't form "*yearer" or "*more year".

The expression "years old" is idiomatic. You can say "10 metres long" but not "10 kilograms heavy". The word "years" doesn't modify the adjective "old". The noun phrase "ten years" modifies the adjective "old".

The phrase "ten years old" is a phrase, not a word. Phrases don't have "parts of speech" (so you can't describe a phrase as an adjective). It doesn't modify "boy". It is a complement to the verb "is". Complements can be noun phrases, adjective phrases or prepositional phrases.

You can't say a word is an adjective because it modifies a noun. Nouns can also modify nouns (eg "chicken soup" - "chicken" is a noun adjunct, not an adjective)

"Ten" is a numeral or number. Some classify numerals as a type of determinative, but they are rather different from the usual determinations like "the" or "my".

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