Mayer's upside is promising: rave reviews as a hitter and fielder, and reportedly yet to lift a weight. Imagine his development on a professional training regimen.
As for when we could see him, remember age is just a number. Duran is 24, but really progressed the past two years. Jackie Bradley, a much more polished outfielder, made the majors when he was 23. Betts, Benintendi and Moncada made MLB debuts when they were 21. Casas is 21 and playing with big leaguers in the Olympics.
Bogaerts and Devers both starred in the postseason when they were 20.