Reading 101. No one said MacKinnon is 5 years older. Go back and re-read.
You can choose to ignore development curves at your own peril. There's literally 100+ years of data at your fingertips. If you don't like data, suit yourself but odds are you'll end up on the short end. Just so you know, here's how both players compared through their first 4 years:
G/GP
0.56 Matthews
0.25 MacKinnon
A/GP
0.45 Matthews
0.43 MacKinnon
Pts/GP
1.01 Matthews
0.68 MacKinnon
At every step, Matthews has been better (and significantly so). MacKinnon is better TODAY but there is literally nothing to suggest that by the same point in their development, the same will hold true. Matthews is just now for the first time getting increased minutes, increased PP and some high level linemates.
Both are great players. Today, I might take MacKinnon but not sure I would over time. Can't go wrong with either though.