I think KA is right in a small way - you do have to realistically evaluate how you are going to fit children and career and everything else into your life. This does not however mean girls having babies as early as possible.
I went to university at 18, got a job with a magic circle law firm and then reflected whether I wanted to go down the partnership route and sell my soul or do something else.
I adjusted my career slightly (although remaining in law). Gave up the prospect of partnership but went down a route that allows me to work flexibly.
I got married at 25. At the time, niether DH nor I wanted children. However, when it came to considering them too, as I was the higher earner, we agreed that he too needed to consider how he was going to structure his career to accommodate them.
I had my children at 33 and 34. Dh and I have juggled them between us.
We have a great life and are lucky. However, we did both make sensible choices about what we wanted and what we could afford.
I think this is the message that young people should be given. Consider your whole life plan early on. Not in detail, but have a sense of when you think you would like to do things and then decide are you are going to manage that.