Dembele was arguably the best player in Germany last season together with a few Bayern stars like Thiago and Robben. After he totally dominated in France. If you're good enough, you're old enough. Just ignore his age and look at his performances.
Name 1 bought attacker aged 18-21 who made it here (Ok, Ronaldo R9).
On the other hand, we had Saviola, Simao, Quaresma, all "future leaders and BallonDOrs" who failed.
Only somewhat youngers players who made it are Neymar (even though his case is extremely tricky and debatable) aged 21 and Ronaldinho aged 23.
So, imo again, not only that it is very questionable whether Dembele will make it here. but also, even if he will make it, he won't reach star-level until 2019 or 2020.
Him turning into a good player would be awesome for our long term future, but again, I don't see how he will help us short term, in Messi's final years and in the days when we have the most expensive (and broken) attack ever, called MSN.
I will repeat again, in the last few years, our game plan was 433 and: pass the ball to Messi or Neymar, and they will dribble past 5 players and try to create something.
And now, instead of fixing our midfield, our new game plan will be 4231 and: pass the ball to Messi, Neymar or Dembele and they will try to dribble past 5 players and create something.
We won't fix the MSN shit, we will reach even more dumber level where we will play with 4 attackers, and where 3 out of them are crazy individualistic dribblers.
And behind them, we will have Busi-Raki, Busi-Roberto or Busi-Gomes.
On a good day against poor teams, we will have 40 dribbles per match and we will be winning 6:1 and 7:1.
But against better teams, our teams (with this lineup) will be cut in half (attack and midfield+defense) with zero control.