Why cant you breed at old age in the game creaturebreeder?

Yes, it's unfortuanet, but when creatures reach 20 days old, they are considered "Oldies" and can't breed. Why? I was hoping this was obvious, but they are past the breeding stage. They can't breed young or old, but only when they are an adult, just how young children can't have children and how your grand-parents can't have anymore kids.