I found a recall bulletin from Trek that says that a final serial number character of "D" represents the 2009 model year. Going forward from that (and skipping "I" and "O" as many manufacturers do because they look too much like 1 and 0), I get this, which matches up with 'R' representing the 2021 model year:

D - 2009
E - 2010
F - 2011
G - 2012
H - 2013
J - 2014
K - 2015
L - 2016
M - 2017
N - 2018
P - 2019
Q - 2020
R - 2021