Anyone over eighteen years of age counts as adult.凡是超过18岁的都算成年人。
Children are usually more natural in their manner than adults.孩子们的举止通常比成年人自然。
The bear was a fully grown adult.那只熊已完全发育成熟。

［派生词］ adulthood n.成年；成人（期）
［同义词］ adj.  full-grown, grown-up, ripe
［反义词］ adj.  immature, infantile, juvenile
［词源］＜拉丁语adultus(长大成人)

1.adult用作形容词的基本意思是“成熟的”,指人身体已经发育成熟,也可指智力及情感已成熟,用于物则指达到发展过程中的成熟阶段。
2.adult也可作“成年人的,适宜于成年人的”解,指已经度过青春期或已经达到法定年龄。
3.adult作“成年的”“成年人的”解时不用于比较等级，作“老成的”解时可用于比较等级。

用作定语
▲～+n.
 We should do our best to develop the adult education.我们应当努力发展成人教育。
I like his adult approach to the problem.我喜欢他老成地处理这个问题。
用作表语
▲S+be+～+prep.-phrase
 He is adult in behavior.从行为上看,他已成熟了。
▲S+be+～+to-v
Aren't you adult enough to know that there are two sides to every issue?长这么大了,你难道还不懂每一件事都有两面性吗?