Note that we don’t know that this particular 8 year old actually has played CoD. This is a list of his goals, after all. Maybe his responsible parents won’t actually let him play it (whereas Ethan and Matthew’s irresponsible parents will).
Having said that, I feel like every kid is different. A good parent should know when their kid is old enough to handle graphic content without being traumatised by it. I was reading horror novels and watching horror (and war) movies at age 9 or 10, and the content of those novels and movies was just as bad (or worse!) than anything that I’ve ever seen in CoD (single player - I don’t have the time to get good enough at these games to play multiplayer). Yet, I somehow managed to make it to adulthood (mostly) well adjusted. 8 might be a bit young, but I don’t know this kid.