You got the point for leading a team it requires certain qualities of patience, maturity and composure and that comes with experience and age .
No one can question Mbappe performance he is exceptionally good player and his contribution for France is noticeable. He'll definitely become good captain but with time it's too early for this.
They have been young players that have been captained in the past the likes Fabrigas at Arsenal, Torres at Atletico Odegard at Norway and also Neymar at Brazil. Some of these players went on to great players individually but did they win any trophies with their respective club and countries? I doubt. Leadership should be giving according to succession to allow orderliness in the squad. Also been a team leader is not about just wearing the armband but controlling the dressing room and this is gotten from not just any player.