Witch hunting a minor who got sent porn by a 20-something is not the bright idea you think it is. No matter how shitty of a person the minor is, or how desensitized to pornography they are, they are still a minor and it’s both illegal and immoral to engage with them in any manner that could reasonably be perceived as sexual. It’s not even a little hard to see how sending pornographic art crosses that line.
It’s also not complicated or asking too much of you to block a minor who tries to engage with you in a sexual manner. If you’re someone who thinks an adult should be let off the hook because “the minor started it.” You might need to seriously consider killing yourself.