I've heard of major developmental regressions as late as three-and-a-half. Diagnoses any later than that are usually either "there's nothing wrong with a nonverbal 5 year old" denial, or "he's always been a little different, but we didn't know what it was" at like 12 or 13 or even adulthood for high-functioning kids. I've seen/heard of sudden extreme onset of PANDAS at a variety of ages though, as late as maybe 15.
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