Pro-football-reference.com now has free access to data on pass coverage for defenders from the 2018 and 2019 seasons (PFF has similar data but it is behind their pay-wall).
One of the nice things about the PFR data is that it shows the age of the players at the start of the season. There were 273 players who had enough coverage snaps to qualify in 2018 and 263 players in 2019. In 2018, 37 of those players were 30 years old or older. In 2019, 32 of them were 30 years old or older.
This got me wondering about how long “old” defensive backs can continue to play well in the NFL?