It gets even crazier when you consider a guy like Pavel Datsyuk, who, if I'm not mistaken, turned 24 before the end of his rookie year. So I guess he must have really sucked.
Not all players develop at the same rate, and not all developmental systems are built on the same philosophies. Matthews being very good at 19 doesn't say much about what he'll be at 25.
I think Matthews is going to have a great career. But I think, like Aaron Ekblad, he entered the league much closer to his ceiling than a lot of young players. He was a big, physically mature kid. Gaining 20 lbs is a much bigger deal for someone who weighs 150 than for someone who weighs 200. So I mean, yeah, Matthews had a wonderful headstart to his career, but that doesn't mean we can expect him to someday put up, say, > 2.5x his rookie numbers, the way we can with some players who take longer to develop.