I just turned 40 and my son turned 3 the next day. I quit my job 2 months after he was born (in a terrible economy) to take care of him since my wife had the better benefits and longer commitment to employer. I studied for nearly 6 months while he was just tiny and got my PE results 3 days before he turned 1. It was easy to study when he was so little. Trying that when he was 2 would have killed me.
It's true, kids do tire you out. But I wouldn't trade it. Mrs. CDC and I promised we wouldn't change our lifestyle when he arrived, but that turned out to be a lie. When considering an outing or camping trip I think it would just be easier to stay home. And naps are a good hobby now. The dog doesn't get 5 hikes a week anymore because Mini CDC is too big for a back-pack and too small for a real hike. We do camp a bit still, but not nearly as much Skiing adventures have shrunken to 2 hours maybe once or twice a week and last year that was a lot less because of work and remodeling. But again, fatherhood is worth it. In some ways I wish I had a family when I was younger, but I was even more immature than I am now. And as for being too old once I get my free time back. I wouldn't trade that. I had a ton of fun in my 20's when most of my peers were focusing on their careers. Yeah, I could've earned more money at that time, but I would've pissed it away since I was young and dumb. I would rather live while I was young and retire old because too may times people die after retirement anyway. they never get to really live.
There's a fine line: When you're young you have time and no money and when you're older you have money and no time, find the sweet spot and live it up.
Kids add another chapter to your life. It's like starting a whole new life and it's great. But having said that, I do think you can be somewhat prepared to have kids. So I have to disagree with MA's comment there (and thoughts on having kids in your late 30's).