I think that he is the best player for France and it was thanks to him that France got to the penalties in the final against Argentina this last world cup as without him France would have probably not even a shot on goal on that game.Things have changed now and despite the old players being usually captains now there is a new trend which the captain usually becomes the best player of the team and I think France has made the right decision to appoint Mbappe as their captain.He surely is one of the most talented players in the world and he fully deserve this title despite his young age.
His age is still very young and it's rare to see young players captain the national team because the national team is a little different from the club where in the club because they often play games together, cohesiveness will be easily established but the national team isn't like that, it could be one of the considerations for choosing him instead not only because he is one of the best French players at the moment but also one of the most respected players in the French national team and has a strong influence so the captain's armband deserves for him.