This was part of my motivation for not choosing my parents as well. They are by no means "old" yet, but they aren't young either. I think there is a reason we have children around the age that most people do. When we are young, we have more energy, and our lives seem to sync up better.
I'm not saying that grandparents can't do the job, because that would just be silly. However, it was important to me that the couple I chose to raise my kids should something happen to me or their father be as much like me as possible, and that includes age, life path, and views.
The biggest issue I faced was selecting my good friends and not my sister. My parents were not nearly upset about my decision as my sister and her wife were. We worked past it, and they came to see why I made the choice that I did, but it wasn't an easy conversation to have with them.