As a modifier, use the singular.
A four-year-old boy.
A 20-year-old whiskey.
To simply say "I'm 20 years old" then use the plural (except for 1).
She's 20. She's 20 years old. She's a 20-year-old woman.
He's one. He's one year old.
When referring to children, we do sometimes say " I have a four-year-old and a nine-year-old." In those cases, the