Can numbers be treated as nouns?

A noun is a person, place or thing. In the sentence, "I graduated in 2015," the word"2015" refers to a year, which is a thing and so is a noun. If you said "He is a 2015 graduate" then "2015" is an adjective modifying "graduate."

Two of them fell asleep under a tree while the third built a fire and stayed alert.

Numbers sometimes function as nouns, as do two and third here, instead of as adjectives. ~You can tell they are adjectives, not nouns, if they describe a noun, as in "two soldiers," which these do not.

This was the best explanation I could contribute...got it from my kids fix-it! grammar textbook.