Again, what Chum is describing is merely a norm, not an absolute, IMO
Something important that people keep leaving out, It's entirely possible for a happy and healthy marriage to exist without having kids. People keep saying that if you have a good marriage and you are happy, you'll be fine with kids. Not true. Some people just don't want to have kids and shouldn't have kids. A healthy marriage can be strained if one person's heart isn't into it - 'cause there are moments when you're going to want to shake those little fuckers. Resentment will grow 'cause your significant other talked you into something that ruined your life. And now you have this needy thing screaming at you when all you're trying to do is help him/her. Or the other person isn't willing to get up and help at 3:00am with a sick kid or whatever etc etc.
With all that said, I wasn't ready to be a father until I saw my sons head pop out. I think it was the first non-sports related time I cried from happiness. It was still a hard transition not staying up all night drinking/playing video games/going to the movies 2-3 times a week and there were times I was a horrible father and a horrible husband. I never did anything to physically hurt my kids but ignoring my wife and crying son while I stayed up all night dicking around did nearly as much damage. Luckily my wife and I have open dialogue and she told me I was being a horrible father/dick head. That was the moment that caused me to change my ways because I actually wanted to be a father and wanted to have kids.
If it matters I was 25 when my son was born; looking back I think it was a little early, but I wouldn't have changed anything. I was always the type of guy that didn't grow up until I needed to and in any case wouldn't have been ready for kids until it happened.