As said in the other thread, my 2006 battery is still going strong. I don't think a battery should be replaced preemptively based on age.
There is a simple load test that any car electrician can carry out, it involves connecting a load tested across the terminals, and then repeating the test 30 minutes later to see if there is any significant performance drop.
Said that... the battery will need replacing at some point, and if you are happy to do this now then this is also fine. My point is that age alone is not necessarily a factor.