Over thinking. Chris is emotionally delayed and has no real measure of the passing of time in his life. No work anniversaries, no traditions, nothing.
Previously, girls had to be 18-CWC's age because that's what he started with and he just kept it going as he aged. He raised it to 21 when he got called on it. I doubt that at the time, or even today, that he has any sort of knowing preference for significantly younger women, and he gravitates towards them now because of shared interests.
It completely goes over his head that the overwhelming majority of adult MLP fans, as an example, are at least 10 years younger than him. He just sees autism horses and drawing and thinks "Gee, she's soo much like me!".
Any time you want to analyze something OPL has done, the first question is "What's the laziest option?". It requires effort to "age up" your potential dating partners as you age. Same reason Chris has never been interested in anyone older than him. They'll expect him to act mature and be interested in more sophisticated things, and that's effort, too.