如果你想表达一个人很年轻，你可以说他是 "very young" 或者 "quite young"。你也可以说他 "looks young" 或者 "appears young"。例如：
He looks very young for his age.
She appears quite young, but she's actually in her 30s.
你也可以使用年龄的数字来描述一个人的年龄，例如：
He's only 20 years old.
She's just 21.
希望这能帮到你！