Wh- Question for "as young as five"

Question

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?

Answer 1

How old should a child be before taking this test?

Answer 2

Strictly speaking.

1: Children as young as what age are able to take the test?

A: Children as young as five

2: Are five-year-old allowed to take the test? [or] How many years old must a child be to be able to take the test? [or] How many years old are the children [who are] able to take the test?

A: Five-year-old children are allowed to take the test.

These question best mimic the structure of your answers. However, they're not particularly natural ways of asking those questions.

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

This sentence is incorrect. It can be corrected different depending on what you want to ask.

At how young an age are children able to take the test?
How young are children [who are] able to take the test?

By the way, when you say:

"The answer to the first would be "At five." and to the second "Five." Not "Five-year-old children.""

I hope you realise that there's no need to respond to questions exactly as they were constructed in English. If someone ask "What the youngest age children can do this?" and you say "Children as young as five can." that's perfectly good English.

Answer 3

In order to justify the "young" in the answer, the question ought to contain it as well. So

How young a child can take the test?
- As young as five.

I am not convinced about your How young an age are children able to take the test? because, strictly speaking, age cannot be young; only a child can.

As for your second question, a similar construction offers itself:

Persons of what age are able to take the test?
Five-year-old children.

There is however no guarantee here that the answer won't be (the perfectly equivalent)

Children of five.

Additionally, I want to remark that there is no way to frame a question in such a way as to guarantee in either of your questions that the answerer will not use a complete clause, i.e.,

How young a child can take the test?
- A child as young as five can.

In fact, I find it more natural to expect that the answer will be a complete clause (with a verb in it).

Answer 4

I've substituted 'can' for 'is able to' in the sentences. Maybe I'm being simplistic, but I think the question in the following format is the most straightforward match to your second example:

Q Who can take the test?

A Five year old children can take the test.

or, if it's a matter of choosing from differing ages of children-

Q Which children can take the test?

A The five year old children can take the test.

It's interesting to ponder, but I think as someone else also said, grammatical structure and what sounds natural can be different from language to language. Also, to me, the following sounds best for your first example:

Q How young a child can take the test?

A A child as young as five can take the test.

So, it sounds better in singular form; I'm not sure why but the question 'how young a - ?'only sounds good in singular form to me, so it follows that the answer sounds most natural in singular form too. ps. This is the same format as your 'How big a TV do I need?'(I'm actually not sure whether you can use plural with this structure correctly; but I've just never heard it).

Alternatively, if you want to use the word 'age'-

Q At what age can a child take the test?

A (A child can take the test) at (age) five.

or-

Q At what age can {the} children take the test?

A ({The} children can take the test) at (age) five.

This is the same format as 'What size generator do I need?'. In this format, you can use both the singular and plural - they both sound fine. You can choose to omit the (bracketed words) for convenience. Choosing to include {the} depends on whether it's general or specific children you are talking about.

So the pairs of sentences above all give slightly different connotations - it depends where you want the emphasis of meaning. But personally, this is how I would match the questions and answers most simply.