Whilst Pogačar was the youngest winner for some time, at 21 years and 365 days, I should point out that Fausto Coppi won his first Giro d'Italia at 20 years and 158 days, and Giuseppe Saronni won his first Tirreno–Adriatico at 20 years and 175 days. There were even earlier winners, way back.
Before Graham mentions it, I should say there were 13 years (and World War II) between Coppi's first and last Giro d'Italia wins. In the current era, Mark Cavendish has won sprints in grand tours over a 15 year period between 2008 and 2023!