Use "who are" in a sentence that is not a question

Question

I know that we use are in present continue in questions: who + {is/are} + {verb + ing}

Can I use who are also in a sentence that is not a question?

I think you should not talk to guys who are not your age, they could not understand you.

If yes, why?

Answer 1

The answer to "can I use "who are" in a sentence that is not a question?": YES

I think that the entry in the Cambridge Dictionary is very informative and it got a lot of examples of that particular use

who pronoun (USED TO REFER)

used as the subject or object of a verb to show which person you are referring to, or to add information about a person just mentioned

I think it was your dad who phoned.
She's one of those people who love to be the centre of attention.
He called James, who was a good friend as well as the family doctor.

But you asked particularly about "who are". Some examples that I've found in a quick search

Announce an immediate and unconditional amnesty for all those who are imprisoned for political reasons.

We represent all the citizens of Europe not just those who are organised in NGOs.

Your sentence

I think you should not play with guys who are not kick the ball like you

is not correct. "are not kick" is not a valid construction.

This one is correct

I think you should not play with guys who are bigger than you

Maybe what you were trying to say is

I think you should not play with guys who don't kick the ball like you

do is the auxiliary verb used to negate kick

Answer 2

Since being of a particular age is a state, not a process, we use the simple tense, not the continuous.

She is seventeen.

not

The toddlers are being fourteen months. ^NO

We ask with the simple tense as well:

Are you twenty-one?
-- Yes, I am.