The fact that Marrone is 22 doesn't say anything, we know how things works in Italy, youngsters rarely gets the chance to play no matter how good they are. Marrone was with Conte at Siena, played so many games 20 IIRC and played a big role in the promotion ( @Salvatore ).
Also last summer Conte made it clear he wanted Marrone, and even with interest from clubs like Fiorentina he refused to let him go, that would tell you something.
Marrone also captained Italy Under 21.
Pogba playing instead of Marrone as Pirlo's backup is a mystery for me, Marrone is the better passer, with more vision, great technique and very good defensively.
Maybe Raoila made sure Pogba will feature for a number of games for him to sign the contract. If not the fact that Conte is holding to Marrone and trying to play him in other positions wil tell you how high he thinks of him.