Looking on the internet deeply has found these results:

ask for his age is the most popular phrase on the web.

ask his age 189,400 results on the web	ask for his age 12,020,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	But he was awake long enough for a doctor to ask him his age. I didn't ask him his age, Francis. May I ask your age, sir? Paragraph 7 asks that age be taken into account as a criterion of excellence together with family situation, including the number of the researcher's dependents. You never ask a lady her age, weight or SAT score. Amy, ask him what age he is. [Whispering] If they ask your age, say sixteen. Then you ask her age her favorite food...

ask his age

189,400 results on the web

ask for his age

12,020,000 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

But he was awake long enough for a doctor to ask him his age.
I didn't ask him his age, Francis.
May I ask your age, sir?
Paragraph 7 asks that age be taken into account as a criterion of excellence together with family situation, including the number of the researcher's dependents.
You never ask a lady her age, weight or SAT score.
Amy, ask him what age he is.
[Whispering] If they ask your age, say sixteen.
Then you ask her age her favorite food...