I was diagnosed at 29. It took me quite a few years between getting that initial sense of recognition reading testimonies from other aspies to getting the actual diagnosis. Partly because so much of the info out there, as well as diagnostic centers and organizations, is aimed not at autistics, but at people who have to 'deal' with autistics. Which also explains much of the (temporary) frustration some of us feel towards these organizations and the medical/care industry as a whole.
Going over some of my teacher reports from kindergarten and the first year of middle school, I guess it could've been noticed a lot earlier. I faintly remember my parents turning down the advice of a speech therapy person who I had to see at school a few times regarding a stutter, who voiced the suspicion there might be more going on than just a stutter. They probably didn't want the interference, which would be in line with their blatant disregard of anything else teachers would say about me, be it good or bad. Well, apart from the bad sometimes. They just weren't ready to accept one of their children having something like ASD. They still aren't accepting btw, so not much has changed. (There is a difference between being accepting, and just being in denial. The first requiring effort to be made, interest to be shown, the second just being lazy, also that might sound a bit harsh.)
Taking into account the public opinion on and knowledge about autism back then, I also don't know if it would've made much of a difference, or that I would've been that much better off with having an early diagnosis. It's only been the last 10 years or so that these opinions have been moving towards a more positive attitude. And still, having an official diagnosis early on doesn't guarantee better treatment. Reading about the ways some parents deal with it, having their children go through disruptive 'therapies', I'd say sometimes it's better to stay under the radar until you have some modicum of self-determination. I hope those cases are getting rarer though. And having an early diagnosis, in combination with a proper environment of acceptance and support, at home, at school, does, in my opinion, make a huge difference later on, if only in terms of self-esteem, not having to feel weird or growing up with the notion that there's something wrong with you, or that you need to be changed somehow. Going over some of the more horrific stories about children being diagnosed young, and the way they are subsequently treated can cause me to cry out of despair, but at the same time, reading about how some of the parents here, and elsewhere, deal with it, giving that much needed support and acceptance, makes me cry with joy because I know those kids will be so much better off.
