Since I was first learned English, I know that any number greater than 1 comes with s for plural (except for some exceptions like Mice and Feet). 6 Cars, 2 Trees, 3 Dogs, 9 Years
So if someone ask me how old are you I will say 6 years old
but when I read a report in news website I find they say a 6 year old boy (not years)
so when to remove the plural s?