Many people get confused about when to use hyphens when writing ages. Here's a Quick and Dirty Tip that will help: When the age is an adjective that comes before the noun and modifies the noun, or when the age is a noun, hyphenate.
My eight-year-old neighbor wrote a poem about commas for National Grammar Day.
That 70-year-old with the purple hoodie loves Justin Bieber.
When the age is part of an adjective phrase after the noun, don't hyphenate.
Charlie Sheen is 45 years old
His twin sons are nearly two years old.
Get more tips like this in The Grammar Devotional: