Discovered last night that the battery in my 1998 was over 5 years old. Replaced it immediately. They just don't last that long on average.
Just for an FYI, the way to determine the year and month of your battery's birth date is to look at the top of the negative post. Stamped in the lead are four digits. The top two are the year (My 2000 battery had "00" inscribed on it). If yours has 02 on it, then it was manufactured in 2002, etc.
The lower numbers are the weeks of the year. My newish battery showed "04" (a 2004 battery) and the lower numbers were "22", indicating the 22nd week of 2004 as the born date.
If you bought your Boxster used and haven't had to replace the battery, now's as good a time as any to pop the front trunk, remove the plastic thumb screws, lift off the center plastic cover, and examine your battery to see when it was born. If it's 4-5 years old, it's not a bad idea to replace it so you don't get stranded.
PS - Do be prepared for a thinner wallet... Porsche batteries are well over a $100 bucks, and you should certainly replace it with a Porsche battery so it doesn't slide around. Most non-Porsche batteries (Sears Diehard, etc.) don't have that lip on the bottom that allows for a secure fit.