age of 前面可以加the。at the age of 是固定搭配，意为“在……岁时”。例如：

My brother joined the army at the age of eighteen. 我哥哥十八岁参军。

需要注意的是：“ at the age of+基数词”可以和“when sb. was+基数词”同义句转换。例如：

My daughter went to college at the age of sixteen. = My daughter went to college when she was sixteen years old. 我女儿十六岁上大学。

前面可以加the，例如：

1.Adjust your language to the age of your audience.要根据听众的年龄使用相应的语言。

2.The age of the earth is reckoned at about 4 600 million years.估计地球的年龄大约为46亿年。

3.She entered public life at the age of 25.她25岁时开始了经常抛头露面的工作。

当然可以了，比如 the age of him is nine.他今年九岁了