What part of speech is sixteen in the following sentence She wrote her first book when she was sixteen?

In the sentence "She wrote her first book when she was sixteen", the word "sixteen" is an adjective.

Here's why:

* Adjectives modify nouns. "Sixteen" modifies the noun "years old" (implied), which is describing the age she was when she wrote the book.

* It's not a noun because it's not the subject of the sentence. The subject is "she".

* It's not a verb because it doesn't show action.

* It's not an adverb because it doesn't modify a verb, adjective, or another adverb.

Therefore, "sixteen" functions as an adjective in this sentence.