We replaced our OEM batteries as they failed, one year apart; so one blue top is one year newer than the other one. One recently failed, which oddly enough was the newer one; so it looks like they will never be in synch as to age. The age of the OEM batteries when they failed were 1 year old, and two years old. The blue top was four years old at failure, and the remaining one (still in use) is five years old.
We charge and use only one battery at a time, so the difference in battery ages is moot. Rarely, we have used both simultaneously when both have been partially discharged; and we were trying to stretch the time before recharging them.