Hachim Mastour is only 16 years old - he was born in June 1998 - but he's been one of the best prospect of the Italian soccer for a couple of years. He arrived to AC Milan from AC Reggiana (a small team that takes part in the Italian third division) for € 850k (almost $ 1M) when he was 14 years old, so Adriano Galliani made a big investment on this young boy and you can see the reason of this expensive move in the following video.
Hachim is a natural dribbler. Or a juggler. Or a magician, you choose. Anyway, if you think he's only 16 years old it's impressive to see what he can do with the ball. But why isn't he already playing in Serie A if he's really that good?
It's mostly a physical issue. Hachim is still not ready to compete at the pro level against strong and rough defenders. Pippo and his staff know that it could be dangerous for his career sending him on the field, especially in a physical league like Serie A where defenders don't care about being "clean" on tackles. So it's important to let Hachim work on his body patiently since he already has the skills to play in Serie A.
Then it's a tactical question. Mastour is a midfielder/forward who prefers playing in a central position, he's not a winger even if their trying to make him play on the left like El Shaarawy. He doesn't fit to the 4-3-3 module with the false 9 - the famous "falso nueve" system - that wouldn't allow him to play at center behind the strikers, but he's the perfect player for a 4-2-3-1 formation or for the nostalgic (do you remember when we used to win the Champions League with Carlo Ancelotti on the bench?) "Christmas tree" module 4-3-2-1.
For these reasons probably we won't see a lot of Hachim Mastour this season, so I think you'll enjoy another highlight video, but this time together with Neymar Jr.
Anyway, even if he will not play in Serie A this season (maybe he will find some minutes lately in the season) he's going to play in Viareggio with the youth team and he'll have the possibility to show his abilities and make some experience in a difficult tournament, even though it's a youth competition.
I see Hachim in a red & black uniform in the near future for many reasons but it's not his time yet, we have to be patient and let him grow.