Serious answer?
Based on my experience working with Autistic children and adults, I would unironically consider a continued interest in Sonic (along with a few other franchises that are targeted specifically to elementary-aged boys; TMNT, Pokémon, etc.) past the age of 12/13 to be a poor sign for further intellectual or social development. I've seen a handful of kids who were still into programs with that core demographic past the age of 13 end up developing into adults with lower support needs, but only very rarely and almost never vice versa. As far as the "Why?" goes, it's probably a combination of factors.
First, it's important to remember that ASD probably doesn't have a single etiology. Instead of being one disorder, it's likely to be several that share some symptoms and genetic risk factors but follow very different courses from embryonic development through adulthood. ASD was initially categorized as a developmental disability primarily because the early cases used to define the category generally showed significant delays in achieving developmental milestones during early childhood. A few adults with lvl. 1 support needs (usually those on the margins) fall into this category, but in general, Autistic children who show serious delays early in life will continue to do so going forward, and will never attain a truly adult mentality. For most people, including atypical adults like those with lower support needs ASD, there are firm boundaries between childhood, adolescent, and adult stages within their personal narrative - the internal "story" of their life that molds their sense of identity. Developmental delay or arrest, especially when it's severe or highly pervasive, can lead to a continued, uninterrupted identity state from whatever developmental stage the individual ultimately attains going into physical adulthood. Basically, as crude as it sounds, they never grow up mentally. Rigidity and resistance to change can lead people with a higher-functioning ASD to retain fixated interests, but generally the way they engage with these will evolve over the course of the life cycle if they're otherwise developmentally typical. If they're not, that's when you start to get into middle-aged men with Sonic posters in the bedroom they've slept in since they were eight.
That's probably the most common form of Sonic-obsessed Autist, but there aren't actually a lot of lvl. 2 or 3 lolcows. This is because, whether for moral reasons or simple disinterest, most people just don't think they're funny. The few exceptions, like Chris-Chan (probably lvl. 2, but actually on the high end for that), Aumist/Zoey (pretty solid lvl. 2), and the chocolate pears fanfic guy whose name escapes me at the moment (low end lvl. 2 or high end lvl. 3) prove the rule. Zoey and Chris are both lvl. 2 due to serious comorbidities, with Zoey having severe BPD and apparently enjoying the attention, and Chris having...a lot? Chocolate Pear Guy was initially added without much background information, in part because his obsessive and nonsensical stories read like pedophilic scat porn until you realize that he's so low functioning that he probably wouldn't know if he did have a scat fetish, because he probably doesn't understand anything about the mechanics of human sexuality. His thread is mostly a peanut gallery, and serious attempts at trolling him would likely be received very poorly. Weird but competent adults rage yelling at a webcam are funny. I'm no saint, but the idea of a 30-something year old with an advanced degree in the social sciences and close to a decade of work in behavioral health and in-patient psych swinging that around like a 9-pound hammer to make a man with the IQ of a 6 year old scream-cry and give himself a concussion is just fucking sadistic. Definitely not something that I have any interest in doing (I really don't fuck with lolcows anyway, unless they're voluntarily participating and/or in a position to fight back; I mostly just do commentary).
Most lolcows obsessed with Sonic fall into a different category. Their development isn't totally arrested, but their social functioning is too low to understand what behaviors are and are not age appropriate. They just don't really experience cringe, and haven't been taught to mimic socially acceptable behavior. So, fixed, rigid interests from childhood remain, but with some alterations in engagement. In this case, that often means jerking the rooster, flickin' the doorbell, muffling the hamster...idk, but they do it to furry porn. These typically come to the attention of sites like this one due to their extremely eccentric tastes in fan art/fan fic, and complete the transition to full-on lolcows by revealing themselves to be argumentative, terminally online degenerates, probably due to the same developmental environment that led them to think that drawing Sonic railing Knuckles up the ass was acceptable in the first place.
