year or years

First things first:

When we tell our age, does the word "year" take an "s"?

As for your question:

He is 25 years old.

It takes the plural for the simple reason that you are referring to more than one year. You might say: "My son is one year old today." A year from now, you would say "My son is two years old today."

Thank you for tour help.
Have a good day.