I had the same problem. I wanted birthdays from birth on, for similiar reason. I set manual birthdays from birth on (only for realtives & family) and deleted the ones from the contacts. It had been nice, if there were an option to choose, when to start birthdays. But now, I have my database, no more need for this.... I think my family will not grow this fast
I just wanted to show, that there are more people, potentially using this feature.