what is the difference between year old and a years old why does it has s sometimes?

If you use a noun as an adjective, it must be singular. A brush that we use on our teeth is a tooth brush.

You can say, "She is five years old." But if you use that phrase as an adjective, it becomes, "She is a five-year-old girl."

Another example: The ceiling is ten feet high. The room has a ten-foot ceiling.

when you use a phrase as an adjective, the noun in that adjective phrase is singular. when the same words are used not as an adjective, the noun is plural.

He is three years old. The three year old boy threw a ball.

The oil is stored in a 250 gallon tank. the tank holds 250 gallons when full.

Some cowboy hats are called "ten gallon hats". I guess that is because they supposedly can hold ten gallons of water (although that would make a huge hat).

One trick to understand this is see if there is any one of the articles(a/an/the) in the sentence. If there is then year stays year ie.singular & if there isn't then year changes into years ie.plural.

Its not supposed to have an s. The people who say "years old" are grammatically incorrect.