It's instructive to look at people who drive for a living. One arm will have significantly more sun exposure than the other and it's trivial to spot the difference as the sun does have a very noticeable aging effect.
Yes - their whole body would be subject to the ageing effects of the sun.
> It seems unlikely that we would have evolved to have a single part of our body exposed to the sun while the rest wasn’t
And yet it's common for dark skinned people to have significantly lighter palms and soles of feet as they have less exposure to the sun.