This might not be universally true soon. The Netherlands proposed changing the law so teens are okay sexting one another as long as there is mutual consent and it's not to make money[0]. However, it has been stalled for a few years now. (I initially thought it was accepted, but now things are forecasted to be 2024). I also suspect that despite it being illegal, very little action is undertaken while the above requirements are still met.
> Mutual, consenting sexting between two teenagers can't possibly be illegal, can it?
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.
> Mutual, consenting sexting between two teenagers can't possibly be illegal, can it?
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.
I do not know about other EU countries.
[0]: https://www.mediawijsheid.nl/veelgestelde-vraag/is-sexting-s... (Dutch)