Onset is when symptoms can be observed. In practice (and phenomenologically speaking) it's the same as "when it starts affecting their behavior in any way", si

Onset is when symptoms can be observed. In practice (and phenomenologically speaking) it's the same as "when it starts affecting their behavior in any way", since we can't say it affects it if we can't see any sign of it affecting it. Unless we can observe or measure (and unless we do observe or measure) we'd just be assuming that "of course it does". Which might be a good assumption, but it would still be an assumption.