by the age of 什么意思

by the age of
到…年龄
age of marriage {名}结婚年龄 {名}
by the age of到…年龄
to come of age {动}到法定年龄 {动}

到…岁为止
at the age of在…岁时
by the age of到…岁为止
die at a very young age年纪轻轻死去

例句：By the age of seven he could write beautifully.
他七岁时字就写得很漂亮了

可以去爱词霸，我常常在那里查东西

多少岁的时候

By the age of 20, Sir Isaac Newton found a general binomial theorem which lead to the later development of calculus.
20岁的时候，牛顿发现了一个二项式地理，为以后发展微积分做好了准备。

He had had 3 kids by the age of 30.他到他30岁的时候已经有了3个孩子

到……的年龄的时候，如：
一般将来时：I will get married by the age of 30.
我会在30岁时结婚

过去完成时：He had had 3 kids by the age of 30.
他到他30岁的时候就已经有了3个孩子。

到什么什么的年龄

By the age of seven he could write beautifully.

他七岁的时候写字就很漂亮了