Teammates are irrelevant to his production if he is not playing with them.
More accurately, the time-frame is usually around 24-26 for forwards, with not as much drop-off for players of this level, especially through their 20s. Matthews is 22. There is no reason to believe he has peaked in ability, and there is so much room for increases in opportunity.
He paced 55 goals and 94 points this year, in a relatively down year, before his prime, without the opportunities that most others get. Small samples, but he paced 58 goals under Keefe while averaging only about 3 minutes of PP time per game, and 63 goals with Marner on his line.