Lets not forget, we haven’t heard HIS side of the story yet.
One way or another, every word they typed to each other is on a server somewhere, and well be subpoena’d if it can prove his innocence.
It’s true. He may have assumed she was 18 because she was on that site, if that site had a rule about ‘no minors’.
On that basis, he may have thought he was meeting an adult.