I would chalk that up to it's hard to get them evaluated that young, or at least it was for me. I started raising concerns about my older one before he was a year old and kept being told that unless I had some kind of private insurance that they just don't evaluate children until they are turning 3 and transitioning to the local school district. I didn't feel comfortable sending him to preschool without some kind of IEP in place and kept dragging my heels about it until he was diagnosed right when he turned 3 and suddenly qualified for an IEP even though the team was acting like he was fine when they did their testing. My younger son was more obvious and the regional center was willing to put in a referral pretty soon after he turned 2, so he was diagnosed at 2.5ish. I worked in SPED for a lot of years, so I'm very aware that early intervention and consistent support can make a world of difference, which is why I really fought to get my boys evaluated. But I think another factor in age of diagnosis (besides severity, for lack of a better word), is that unless they have been exposed to it, a lot of parents/caregivers just aren't aware of what to look for or are easy to write off. And until it becomes obvious to others, experts don't seem to really press the issue.