Do you mean that you want the older person to be listed on the left when two people are married, or that if someone is married more than one time, the order of partners should be from left to right by birthday?
I believe the default is to have the mother on the left and the father on the right. I think this is done to make tracing geniologies by either the mother's or the father's line easier. I didn't actually notice that until I looked into your question. It seems like this default behaviour is probably best.
