I assume it's anywhere under age five. But that can be very difficult to tell if the kid is so mild. Things kids do at a young age is actually normal but with autism, it's more extreme and if someone is mild, their symptoms may not be noticeable because it would blend in with being a toddler so therefore their symptoms would be seen as normal. My son is always watching Spiderman and that is pretty much he wants to watch and it's all day long. He is always playing with cars too and it's usually the same things over and over and he will ask if we are happy. That is normal at that age. Toddlers do have narrow interests so they will be fixated on one thing. If an aspie was doing it at that age too, it would also look normal to me unless their symptoms were that extreme but if theirs is mild, I don't think it would be noticeable.
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Son: Diagnosed w/anxiety and ADHD. Also academic delayed.
Daughter: NT, no diagnoses.