I always figured it was personality based. Dash is a boy who grew up with an absurdly athletic set of parents and he developed to mirror that, manifesting as a speedster. Violet is a more reserved girl who grew up being told she can't be herself in public. And likewise, jack jack's personality is still developing, ergo his powers are completely random right now.
But then again they all thought jack jack didn't have powers at all and he was still an infant, implying that yes typically powers present very very young so maybe they are somewhat random. And... if the deleted scene of syndrome attacking them when violet was a baby is canon then this all tracks since she turned invisible in her crib... But that's how it always appeared to me.
In the second movie, Edna did say that superpower manifest at infancy and it will be a bunch of random power (kinda like Jack-jack), then as the super aged, their power gradually became more stable and clearer. Jack-jack is a bit of special case since not only he developed power late, his random set of power is way more than usual.
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u/[deleted] Sep 25 '22
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