2 years is too short to start showing any decline in performance. I can usually tell a slight decline in batteries 2 years apart in age by their resting voltage, but performance is still nearly that of a new battery. About 5 years seems to be the time most start showing a decline that I replace them(you can still use them but their storage is much lower).

I'm wondering if you may be overcharging in some way and causing problems? What about winter storage, do you leave them alone or keep them on a charger? How often do you have to add water?