He is a three-year-old boy.=The boy is three years old.两者意思相同,简单地说,three-year-old 可以看做是形容词,后面必须要加上名次.注意：three-year-old 中加了“-”,years后面的“-s ”一定要去掉.同样的例子还有two-day off(两天休息日）