What he means is that brain is still developing in our 20s. I believe until 25 in boys, until earlier in girls. But if you take his line of reasoning, then 18 year old girls should not be allowed to drive, vote, babysit and hold other jobs where they make important judgment calls, decide which educational avenues to pursue, and so on and so forth. Is he prepared to live in such a world? Can he see any challenges in operating such a world if he were given the reigns and the title of the benevolent dictator?

Since he is autistic and apparently high functioning, he may well appreciate an opportunity to reason about these scenarios in the abstract instead of attaching emotionally charged pejorative labels, such as child predator.

I would try reasoning with him before you decide to block him. You can always block him later. There is no rush