Thats because he's been around for a long time. He Was a great player at PSG with 18, joined Arsenal at 19 for just 750000 €, and was sold to Real at 20 for 30+ mil €. Quite a profit Arsenal made on him. After a year in real he went back to PSG, where he spent half a year, before going to Liverpool. he spent half the season there on loan, and joined Manchester City after that, two years ago.
he's young, but he's changed a lot of clubs and has plenty of experience in the top.