I think it’s been touched on in a few places, but a very big factor I think is the power meters and data that we have now (or more importantly, that the teams have access to now) that can tell the DS that “hey, this kid is actually your strongest rider, by a long shot too,” where before a 22 year old who came into the peloton would have to “serve his apprenticeship” and “learn his racecraft” by fetching water bottles and eating wind for an aging team leader pushing for a top 10.
It would have to be a truly out-there generational talent like an Ullrich or Armstrong to break through in that system and get team leadership aged 21 (and additionally, be a German/US rider, with a German/US team, specifically looking for that talent).
Case in point, the ‘83 Tour. Fignon was due to be Hinault’s water boy, until Hinault withdrew. And Roche and Anderson had to give up their own chances working in a French team for a French leader who was injured, and never repeated his performance again.
Transport Pogacar‘s career back to those days, and instead of leadership roles at GTs he might have spent last year and this domming for Dan Martin, Aru and Formolo. But instead, somebody at UAE looked at his data, said “y’know, the kid has something,” and gave him his shot.