请教各位老师：
复合形容词15-year-old可以单独作主语吗？下面两句话中，复合形容词分别由a和the修饰，两句话都正不正确？
A 15-year-old sometimes forgets things, but that doesn’t mean hisor her memory is getting worse.
The 10-year-old gets up half an hour earlier on school days to playcomputer games.
A 15-year-oldsometimes forgets things, but that doesn’t mean his or her memory is gettingworse.
The 10-year-oldgets up half an hour earlier on school days to play computer games.
句中的 15-year-old和10-year-old 不是形容词，而是名词，自然可以用主语。当这种结构用作名词，它还可以有复数形式：15-year-olds 和10-year-olds。
