I got my first ones in my calves at 10, I was in no way fat, but since my skin is extremely prone to cuts, bruises, stretching, anything, I got them. They were flaming red and yes Nygeel is right, they are scars (so I thought they were cool...).
I have them on my calves and thighs, but they have faded to pearly white and since my skin is very light, they aren't very noticeable. My sister has less of them, but since she's swarthy, they're a lot more noticeable.