Earlier quoted context omitted.
If they have fewer kids then where's the harm?
This is very true. It's proven itself over and over in history. The more stable, healthy, and developed a country is the less children it has. Europe and East Asian countries are having this problem right now as both have negative population growth. In fact this might be the solution to over population. If I knew I'd live to 150 I'd probably wait till I'm 75 to reproduce and replace myself with children.
The cultural issues could be pretty huge if women are only fertile for a small fraction of their life and dudes are fertile for X times as long as women. Or maybe it becomes a cultural "job" of young people to churn out kids as fast as possible for old people to raise them, which we seem to already be implementing in certain socioeconomic classes.