Coppi won his first Giro at 20. At 22 0r 23 he broke the hour record. He would have had a few more impressive early wins but there was a bit of bother in Europe at teh time.
Merckx was 20 when he won Milan San Remo for the first time. By his 23rd birthday he had won it again and added a Worlds, Paris- Roubaix and a Giro. In htat Giro he picked up all 3 major Jerseys. No one else has ever won all 3 (no not all 3 in one year, all 3 at all)
Anquetil picked up his first TDF at 23.
Hinault did the Tour Veulta double as a 23 year old. By that time he also had at least Leige-B-Leige.
Looks like at 25 and 26 these guys have some catching up top do.