One study I looked up said drones begin drifting when 6-7 days old. The proportion of drones that drift increase with age to a level of 50% by 15 days old (50-60%); they prefer drifting to colonies containing virgin queens.  About 20% will drift more than once.  At distances greater than 50 meters, drone drift decreases and not much drift after 150 meters.  
Drift certainly could account for a big portion of what I've observed in my hives, but pretty certain I recall seeing both light and dark drones emerging from cells earlier this spring.

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