wearing a battery down seriously damages it, they are not designed to take that.. one day you'll notice it won't crank the engine anymore, even after charging..I wore my original battery down a few weeks ago in very cold (-10F) temps. I checked it a few days ago and the life indicator was blue. She turned right over. Glad I didn't buy a new one.
the bottom line is, they were damaged by discharging too low, and won't perform as a good battery would. also, make sure these are not deep cycle batteries. they are not designed for starting, and get damaged by a charging current that is too high, most vehicles would use a charge current that is way too high for deep cycle batteries..I am getting some freebee almost new Ambulance Batteries that were apparently discharged too low, and now won't meet certification.
Age alone doesn't mean much without knowing hours, temperature and other conditions.