Payton got his first mark this winter. He only has one so far. I expect him to have a body full by HS, as DH and I are both covered.
I don't know the medical answer, but my personal experience is that as I've gotten older, I've gotten more. Except I now have fewer on my back-- I have 'halo moles' (which is where my skin's been attacking the moles and eventually winning the battle).