Is it five-year-olds or five-year olds?

The rules are:

When the age is an adjective that comes before the noun and modifies the noun, or when the age is a noun, hyphenate:

So:


"My eight-year-old son won a sports trophy." ; before the noun and modifies the noun

"In this country, five-year-olds start primary school." : the age is a noun


When the age is part of an adjective phrase after the noun/pronoun, don't hyphenate

He is 62 years old : after 'he'
His twin sons are nearly five years old. : after 'sons'