What do you care & what does it matter

They all sound natural to me. I would say they all need question marks, but I'm not 100% sure. Perhaps for rhetorical questions, question marks are not required.

We need a second opinion.

Edit. The ones I've looked at all seem to have question marks.

alc241 What does it matter how old he is
2 What does it matter if he comes or not.

3 What does she matter to the company.

4 What do you care if she comes or not.

5 What do you care how old he is.

You need a question mark at the end of each. The third stikes me as unidiomatic. I'm not even sure what it's supposed to mean.

I was thinking the company had treated her inconsiderately, and other staff were discussing it.
That is, What do those

Hello CJ and Avangi,

I had one question:

Can both of these be said?

Who cares who helped you? The most important thing is that someone has.
What do you care who helped you?

Thank you so much

alc24Can both of these be said?

Who cares who helped you? The most important thing is that someone has.
What do you care who helped you?

Yes. They can be said.

(Note that "Who cares ...?" and "What do you care ...?" have negative connotations and can be insulting and demeaning to the person you're talking to. They are dismissive of the

CalifJimThey are dismissive of the person you're talking to.

I sometimes hear these used in a way which is intended to be dismissive of the third person by way of reassuring the second person.

Jack told me I should have minded my own business. (reply) Who cares what Jack thinks. You were just doing your job!

Even "What do

True. I should have made it clear that my comment applied only to cases like those seen in the OP's examples.

Who cares what you think?
is thus quite different from
Who cares what Jack thinks? (where Jack is not present to hear it, of course).

CJ

We're on the same page. [Y]

Sorry about the edit. I was editing mine while you were writing yours; and then I thought I might as well fix a couple of things.

One question please?

Is it natural to say:

What do you care what people think?

Thank you

It's correct and natural.