I think I express myself badly because I wanted to ask that “s” is necessary to put. Because I learned once that here five-year-old is an adjective or attribute and we don’t have to use “s”. But I don’t find this rule where I had learned it, that’s why I ask you.
I think there is something to what Katy says. I believe it is possible to use five-year-old without the s, but in that case five-year-old would need to be preceded by an article – in this particular sentence, an indefinite article:
My little girl is growing up too fast. I want her to be a five-year-old forever.
If I am not mistaken, ‘‘five-year-old’’ is a compound word that acts like an adjective for the elliptical noun ‘‘girl’’.