Mines 16 years old, went flat over Christmas, left on trickle charge while looked into a replacement, put back on to see if dead, and has been performing perfectly since.
I replaced my original battery after 17 years, simply because I didn't want the inconvenience of it calling time on me one winter.
Having made the change (and I went with a new Varta given the excellent performance of the original) I do find that the car is running that bit better, and I'm getting a slightly improved mpg. So it was definitely time for a change.