A Lead Acid battery, properly looked after and not overworked, will last a long, long, time. Toward the end of its life, performance will drop off gradually. It isn't going to suddenly stop being a battery. If it does that, something else has gone on, and that could just as easily happen with a new one.
A 12 volt battery in good condition should be over 13.5 volts when charged, any less, it isn't charging fully - first thing to check is the charger. But as said earlier, if it ain't broke, don't fix it. Not many things more disappointing that replacing a perfectly good part with a dud fresh out of the box.