Tires take a serious beating—constant "stretching" as they roll along the road, exposure to harsh chemicals (gasoline, oil, acid, etc.), UV rays and heat. That rubber is suffering so that you can travel! Almost all tires will begin to exhibit small cracks in the sidewall after a short period of time. Is it time to worry?
The outer layer of rubber on the sidewall is simply a sealing layer to keep the elements away from the inner (structural) part of the tire—in other words, those unsightly cracks are purely cosmetic. If your tires start to get really deep cracks after 3-5 years, you might consider replacing them for peace of mind. If you haven't worn out a set of tires in 5-8 years, it's worth replacing them for safety's sake, even if there is still good tread left on the tires.
It is important to note that recreational vehicles—like your VW camper, maybe?—are more subject to tire cracking than everyday drivers. Long periods of "sitting" causes the rubber to dry out, and when you take your camper out for a drive, the rubber "splits" as it gets worked on the road. Again, this is nothing to worry about unless the cracks are exceptionally deep.
The bottom line is this: In almost every case we've seen, you do not need to worry—you just need to get out and camp!