Plenty would argue it's very easy to hit good stats in the Bundasliga playing for Dortmund compared to carrying an Aston Villa side. Plenty would argue Grealish carries more influence on the game than Sancho does, and that Grealish is a ridiculous talent, one of the best in the Premier League in fact let alone the Bundasliga.
The argument about age is indeed silly, because players reach their potential at different times and at different levels. As I said, it's very presumptuous to claim Grealish is only better than Sancho right now because he's just a few years older.