If the spots aren't raised, scab-like, inflamed in any way, but instead are just like freckles, then I don't think that's anything to worry about. My previous dog, who was a white dog, developed these freckles both on his testicles, and on his belly. And it wasn't age-related for he had them when he was younger too. It WAS sun-related. He always developed them in the summer, and they faded away in the winter.
Also, they were not related to skin cancers of any kind.