I would guess that he gets a mid-season promotion, perhaps after the All-Star Break. Since, like, 1990, only two catchers have stuck after debuting before their 21st birthday, I think. More have failed. It's just hard to establish starting catchers at any age. Easier to come up as a backup, and learn to hit while somebody else plays the heavy. My first thought when it comes to catchers coming up at 20 is John Gibbons. Doesn't mean it can't be done, of course.