I went to look at the upright stones (http://www.arlingtoncemetery.org/images/ANC_surroundings/PAGES/image39.html) at Arlington, and found pictures of some graves with spouses (http://www.arlingtoncemetery.org/images/ANC_surroundings/PAGES/image47.html). It would appear that not just anyone can have his spouse buried with him. I would guess that if there's any sort of thing about you that's interesting, you can get in - a general, near world record age, whatever. No hard and fast rule about it.

As an aside, I found a pic of Bobby Kennedy's grave (http://www.arlingtoncemetery.org/images/ANC_surroundings/PAGES/image28.html).