It's nothing to worry about. Just don't call him "Moo Cow" as he might feel insulted.
They are not age spots. Little C got them around that age, if I remember correctly. They come and go on her. Depends on how much time she has spent in the sun. The spots have something to do with your dog's pigmentation.
Just hope that he doesn't get one like Sir N's: it's right next to his, um, poo exit zone, and it is dark brown, so it LOOKS like poo is stuck on him if the hair is cut short there.