Learning to play fair

My son is 7 years old, so most of the sports he plays are noncompetitive ... at least in theory. The parents don't keep score, but the kids do. They may not be particularly accurate (hence the many conflicting guesses at the final tally), but they notice who's winning and who isn't.

And of course they notice this when playing games with adults, too. Then the problem is that the grown-ups tend to have a distinct advantage. So is it better to let kids win, so they can experience success? Or let them lose, so they learn how to lose? There are several ways to approach playing games with kids. What's your opinion?