Ritchie is better at the same age but Kassian at age 18 was nothing close to what he is now. He has progressed significantly every year in all facets of the game. He's better conditioned, much faster, more consistent, and stronger on the puck. I'm not sure if Ritchie can improve that significantly between now and when he's 23. Kassian when he has the puck down low along the boards, is close to impossible to move off the puck. I can probably count on one hand the number of players in the league that are stronger on the puck along the boards than Kassian.
I'm not sure if Ritchie can progress as much as Kassian did since getting drafted, he doesn't seem to have as many weak points in his game that he could correct as Kassian did in 09. We'll just have to see how he turns out but if Ritchie is as good as Kassian when he's 23, Ducks fans will be very pleased.