Batteries have a finite lifespan, or more specifically, the number of cycles their plates can tolerate. But normally, a well-cared-for battery should last 7 or 8 years -- but well-cared involves a number of things, including moderate temperatures, a good charging system (the most important thing), checking the electrolyte level regularly, keeping the posts and clamps free of corrosion (to let the charging system do its thing), etc.
And, of course, this is aside from mechanical damage (twisting a post to get a clamp off, etc.).

Assuming an ordinary wet or flooded battery (not one of those Optima or gell-cells), one of the worst things is to let your battery run down to below 50% charge (like leaving the lights on), which is about 12.25v. Sure, you can recharge it from below, but the battery will never recover fully (the plates will be damaged by sulfation) and no longer have its rated capacity or lifespan.
Also, the charging system should start (cold battery) at about 14.4 v (saturation phase), and then reduce the charge to about 13.8 (maintenance phase) -- to reduce overcharging and boiling off the water in the electrolyte solution. These, btw, are cold weather figures (this time of year) -- in warm weather, those voltages should be a few tenths less.

Now, to tell if your battery is healthy or not, do NOT rely on a voltmeter!!! It reflects a temporary(!) condition of the battery at best, just after a charge -- think of it as putting air in a leaky tire: at first the pressure is good according to your pressure gauge, but it will deflate over time.
If you must rely only on a voltmeter, the only reliable way to measure the battery's health is to disconnect the battery (so there's no outside drain) and then take a reading about 24 hours after you've fully charged it. It should, in a healthy battery, be 12.65 or better! Remember (after this 24 hour equilibrium period), 12.65 or better is 100% charged; 12.45 is about 75% charged, 12.25 is 50%, and 12.05 is "dead zone" -- the battery is trash! (These figures are for ordinary batteries -- maintenance free, low maintenance, and "calcium" batteries have very slightly different figures, anyway.)

A much better way to check the battery is with a hydrometer to measure specific gravity of each cell (check all six -- frequently one cell dies before the others). The device will have markings to indicate the condition of the cell.

There's a lot to learn about batteries, but the best sources are at marine websites, because boat batteries (especially the "house" batteries of sailboats/yachts) cost thousands of dollars (and can weigh up to hundreds of pounds), and this investment have to be cared for properly! This involves (in addition to checking electrolyte) such things as three- and four-phase charging systems, automatic compensation for battery temperature, etc., most of which isn't incorporated in automobile charging systems (because car batteries so cheap, relatively speaking).

Good luck. And feel free to write back for more questions.

Happy New Years.