I am an only child (and proud of it), and my DH has an older brother that has spent most of his life in state custody (he's mentally handicapped), so he was brought up as an only child as well.
I have a number of cousins but I'm the oldest. On my dad's side, the cousin closest to me in age is 1/2 my age, I'm 33, he's 16. One mom's side, it's a little closer. I really grew up with the adults, ie. mom, dad aunts.uncles & grandparents, not my cousins. Dad and mom had 3 and 4 siblings each.