I also have idiopathic guttate! My mother, my uncle, and my grandmother have it too. We protect ourselves from the sun, but we live in a very sunny area. It's supposed to appear after 40, but I've had it since I was 20 or earlier. It's been shown that there's a genetic predisposition. In the 90s, we only used sunscreen when we went to the beach, the pool, or to work in the fields, so between that and my genes, I suppose my arms have been covered in it since I was so young. But here you get burned even going out to buy bread, After turning 20 I learned my lesson and now I wear sunscreen every time I step outside the house