cv joints seldom wear out from normal driving. what usually kills them is a ripped boot which lets grease out, and dirt and water in. rarely have i seen a cv joint wear out from age and not contaimination. only one time as a matter of fact, my friends 1.8L jetta which he rallycrosses. high joint angles coupled with lots of first gear made his joints start clicking even though the boots are in fine shape.
for most people, check your boots every couple thousand miles for cracking or rips. if its getting worn, replace it. a $7 boot is alot cheaper than a $50 cv joint.