I noticed a bot team in my league was easily beating all other bot teams, but it wasn't until I played them last week that I noticed something even weirder: most of their players are 50 or older (and their ratings are also quite high).
https://grid-iron.org/club/players/team_id/878
Bot teams do not train, so they don't get any better, but they also don't get any worse as they age. Players on bot teams do not retire either. This ensures the bot teams always have enough players to have a full lineup. While the players age, their stats will remain exactly the same as they were when the team went bot and will remain that way until the roster is dumped and replaced with new players for a new owner.