I think it is one of the best indicators of potential ...when a rider starts winning at an extremely young age.
Armstrong won the World's at age 21. Who the hell does that? We should have known then that he was going to be something very special.
Boonen was how old when he got on the podium at Paris Robaix with Discovery? Wasn't he like 22 then? No one aged 22 gets on the podium at such a difficult race.
Cyclists generally get better with age and reach a peak around age 29-32....so when you see someone winning huge races at age 22 then you best put a mark by his name.