Just for fun, I checked baseball-reference.com to see who Realmuto’s most similar players through age 28 are. The way the do these list, as I understand, is that they consider offense only and do not adjust across eras or ballparks, but they do consider position. Anyway, here is the list. Many of these guys are really not as good as J.T., but the way they do the comparisons isn’t perfect. For some additional information, I included their WAR through age 28, for the next three years (29-31), and the next six years (29-34). J.T. has 17.6 WAR so far in his career.
Miguel Montero (12.3, 3.0, 1.8)
Gabby Hartnett (19.4, 11.6, 25.5)
Rich Gedman (12.2, -0.9, -2.7)
A.J. Pierzynski (11.9, 4.3, 7.4)
Earl Battey (12.2, 5.3, 4.6)
Matt Wieters (15.5, 2.0, 3.1)
Jonathan Lucroy (14.3, 4.5, 2.8)
Mike Lieberthal (8.1, 5.8, 7.4)
Carlton Fisk (22.0, 13.8, 23.8)
Terry Kennedy (12.0, 6.4, 9.7)
The two Hall of Famers, Hartnett and Fisk, had long, productive careers well into their late 30s (in Fisk’s case, into his 40s). The other eight (none of whom accumulated as much WAR as J.T. through age 28) all tailed off significantly, some quite dramatically. Four of them had negative WAR from age 32-34. Two of them never made it to their age 34 season; the two active players are entering their age 34 seasons now, and both are backups. All ten did remain primarily catchers for the rest of their careers.