He can still develop a lot. A players development is not exponential and sky rocketing between a certain age. There are many factors involved in a players development, like the managers and coaching staffs ability to develop a player, learning from influential team mates (like Maddison) and playing in a bigger league like the Premier League.I think if he was a 20 year old doing that it would be very impressive. Just that he isn’t that young how much more can he develop his game to be at a troohy challenging team