old enough to know better 已长大该懂事了；年龄大了解的更多

【例句】
Have you been drawing on the walls? I thought you were old enough to know better.
墙上是你画的吧？我还以为你都懂事了呢。

As soon as people are old enough to know better, they do not know anything at all.
当人们一旦老至应该更加明白事理之时，却是什么都不懂了。