This is also true of cats. My daughter was fearful of having her cat Ashes spayed, and she was almost a year, I believe, when we finally convinced her it was best. Sadly, Ashes did develop breast cancer at about age 8 and a half, and passed away last summer. I believe, and have read several things that confirmed it, that if she'd been spayed at 6 months, she probably wouldn't have gotten cancer.