I think it probably has more to do with numbers of trades and variability/steadiness of results than with years?
It's going to take a swing-trader from weekly charts far longer to feel confident that s/he's a successful trader than someone trading 10 times a day from 2-minute charts (a "semi-scalper", let's say), because the evidence for that belief is clearly going to take hugely longer to achieve statistical significance?
(and more specifically, I think it's right for the "results component" of the definition to be described in terms of drawdown-avoidance rather than in terms of profit achieved - though doubtless some people here will disagree with me about that, too).
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Good points! I resonate with that last point in particular. It took me a while to understand that capital not lost is "more important" than capital not gained (opportunity cost). Mathematically, they are the same but, for a reason I can't articulate well, I agree with the emphasis on "drawdown-avoidance".