▲其实 18 year-old 和 18-year-old 都可以的。像18-year-old 这样用两个连字符的例子很多，这里就不再列举了。下面是几例类似18 year-old 这样的用法：
Ten year-olds are incredibly energetic. 10岁的孩子精力格外旺盛。（《柯林斯Cobuild高阶英汉双解学习词典》）
My four year-old daughter was asleep on the sofa. 我4岁的女儿在沙发上睡着了。（《柯林斯Cobuild高阶英汉双解学习词典》）
More than a quarter of seven year-olds cannot add up properly. 7岁的孩子中1/4以上都不会正确地做加法运算。（《柯林斯Cobuild高阶英汉双解学习词典》）
A 14 year-old absconder from a children’s home in Bristol was found alive and well in London this morning. 一个14岁的孩子从布里斯托尔一家儿童养育院逃了出来，今早有人在伦敦发现了他，他还活得好好的。（《剑桥高级英语词典》第3版）
▲值得说明一下的是，如果所涉及的年龄是一个“年龄段”或“年龄区间”，还不宜在 year-old 前面用连字符。如：
The magazine is angled at the 20-35 year-old women’s market. 该杂志面向20岁到35岁之间的女性读者。（如果写成 20-35-old-year，反而让人费解）
It's a programme designed to appeal mainly to 16 to 25 year-olds. 这个节目的目标观众主要是16到25岁的年轻人。（如果写成16 to 25-year-olds，也让人觉得奇怪）