His youngest son ______ is a teacher.

the answer is " that is 25 "
能否换成“whose age is 25 years old”

不能
that \which在代物时常常可以通用,但有时只宜于用that ,不用which.

(1) 先行词是形容词最高级或者它的前面有形容词最高级时

This is the best that has been used against air pollution(反对空气污染) in cities.

English is the most difficult subject that you will learn during these years.



(2) 先行词是序数词,或它的前面有一个序数词时

He is the last person that I want to see.

(3) 主句中已有疑问词时

Which is the bike that you lost?

(4) 先行词既有人又有物时

The bike and its rider that had run over (撞倒了)an old man were taken to the police station.

(5) 先行词是all,much,little,something,everything,anything,nothing,none,the one等代词时

You should hand in all that you have.

We haven’t got much that we can offer you.

I mean the one that you talked about just now.

(6) 先行词前面有only,any,few,little,no,all,one of,the same,the very等词修饰时

The only thing that we can do is to give you some money.

Li Ming is the only one that got full marks (满分)in our class.

Li Ming is one of the students that want to be teachers in our class.

(7) 有两个定语从句,其中一个关系代词已用which,另外一个宜用that