Just turned 30, so he should be at his peak right now. He is comfortably the best opening batsman in the world, a genuine match-winner who can turn a game single-handedly, and there are only a handful of players in the world that can do that. For me he still has 2 things to conquer in the Test arena, that is India and England away. He is good enough but probably needs to rein his game in a bit, especially in England. When you go as hard at the ball as he does, when the conditions are doing a bit he will be vulnerable early. In 2015 he averaged 40 and passed 50 5 times but no hundreds, so he was nearly but not quite there. India he just needs to bat time and the runs will come.
In England, I wonder if there is an argument for him (and Smith as well) to drop a place or two in the order. Get people with tighter techniques to face the new ball then make hay later in the innings?