同意黎老师的看法，下面是一些相关补充，供参考：

▲用作形容词的语法特征
类似 20-year-old 这样的词用作形容词时，通常只用于名词前作定语，不用作表语。如：
正：He has an 18-year-old daughter. 他有个18岁的女儿。
误：His daughter is 18-year-old.（可改为：His daughter is 18 years old.）
正：All 18-year-old males are required to do a year’s military service. 所有18 岁男性公民必须服一年兵役。
误：All males who are 18-year-old are required to do a year’s military service.(可将18-year-old 改为18 years old)
▲用作名词的语法特征
1. 可以连用冠词
Stop behaving like a three-year-old! 别跟3岁的孩子一样!
For a four-year-old, her vocabulary is very good indeed. 对一个4岁的孩子来说，她的词汇量确实不小。
The 19-year-old will be appearing in court on Thursday where she will face criminal charges. 这位19岁的少女将于周四出庭，她将面临刑事指控。
2. 可以连用物主代词
My four-year-old squabbles with his friends. 我那个 4 岁大的孩子常和他的小伙伴们斗嘴。
I find it very hard to dress my two-year-old when she refuses to cooperate. 我发觉两岁的女儿要是不配合的话，我就很难给她穿上衣服。
3. 可以有复数形式
He was not considered competent to teach seven-year-olds. 他被认为没有能力教7岁的孩子。
I’ve read that you’ve enlisted some 12-year-olds to help out in your campaign. 我读到你让一些12岁的孩子在宣传活动中做帮手。
I don’t think most nine-year-olds have their teeth brushed. 我想大多数9岁的孩子不会让别人帮着刷牙。
4. 可以有所有格
A four-year-old’s attention span is usually about 10 minutes. 四岁孩子的注意力持续时间通常为10分钟左右。
The 23-year-old’s screams alerted a passerby who went to her rescue. 那个23岁女孩的尖叫引来了一位过路人前去营救。
5. 可以受形容词的修饰
Xandra is an athletic 36-year-old with a 21-year-old’s body. 36岁的汉德拉身体健壮，有着21岁小伙子的体能。
His body lacks the bounce and vigour of a normal two-year-old. 他缺乏一个正常两岁儿童应具备的弹跳力与活力。
These tests are beyond the capability of an average 12-year-old. 这些测试超出了一个普通12岁孩子的能力。