If those teenagers are minors and are sending sexual pictures of themselves, they are sending sexual pictures of minors. Without an exception in the law to specifically allow this, that can be and is illegal by the laws that criminalise possession or distribution of sexual pictures of minors. There should be an exception in the law for it, but until there is, there isn't.
It absolutely is.
It’s still consumption of illicit material depicting someone under the age of 18, which is a law being broken, even if it's by someone younger than 18.
Ask me how I know.
I'll give you a hint: I have an absolutely awful sounding thing on my criminal record from when I was 15.
Yes, it can and it is. Though most reasonable places would probably not enforce it harshly.
I think the grand parent comment is talking about leaking those images to other people without permission. That absolutely has to be illegal.