grammar - Wh- Question for "as young as five"

There is this statement:

Children as young as five are able to take the test.

What is the question the answer to which is "as young as five"?

Is this structure correct?

How young an age are children able to take the test?

If no, what is the exact structure? Is it a way of asking that question in English, anyway?

What about this one:

Five-year-old children are able to take the test.

What is the question the exact answer to which is "five-year-old children"

In the following questions

At what age are children able to take the test?
Or
What's the minimum age of children able to take the test?

I know these questions could be answered in different ways, but the exact answer to the first (At what age) would be "At five." and to the second (what's the minimum age) would be "Five."
What about a structure the exact answer to which is "Five-year-old children."

What about "How old children are able to take the test?" It does not sound natural to me.
But I've seen structures like

How big a tv do I need.?
Or
what size generator do I need?

Is it possible to use the patterns in this case and say

How old a child/children is/are able to take the test?