Having a baby changes a person's life in so many ways, regardless of how old you are when you have it.
For a teen or a young person, the changes are obvious. All of a sudden you can't just go out with your friends when you want to. You have this responsibility that totally ties you down.
It is that way for adults too, except that you are older and expect it more. But that is still one of the major impacts. You have to start making your life revolve around what is best for your child. For me, for example, that meant that having a baby meant giving up full-time work to be a stay at home parent because my wife and I felt that was what was best for the family as a whole.
In addition, having a baby changes your whole outlook on life. All of a sudden, you have this huge responsibility and this person that you care so much about. You spend a lot of time trying to figure out how to do the best job you can raising the child to be a good person and a successful person. Just as you have to change what you do on a day-to-day basis, you also have to change your thought process to the point where you are always thinking about what is best for the child and to where you try to live your life so as to be an example to your child.
Overall, having a baby makes us so much more aware of our duty and responsibilities to others and less hung up on our own needs.