The way I try to remember who my first ones were and who the first baby born was is that the game starts with 8 people, including the little baby. Therefore, the eldest should be the first baby born. But I am tired, so I can't think it through to determine if this would ALWAYS be the case or not. I usually end up getting to know the first 8 to 12 villagers really well though, so I normally remember which ones were the first ones. It's after they get more than 12 to 15 that I start having problems knowing and remembering who they all are.

I don't know if that will help you figure it out or not.