I am 18-year-old. ✔
They mean the same, but 18-year-old is a NOUN PHRASE
As a NOUN PHRASE, you can have the plural:
We are 18 yearS old. ✔
We are 18-year-oldS. ✔
"The plural of '18-year-old' as a noun phrase is '18-year-olds'. Modifiers are not modified to correspond with nouns in English, so changing 'year' to 'years' would be an error. In many other languages, the corresponding change would be required." (forum.wordreference )
