I'm 28, married with 3 kids, house, full-time job, etc., and doing a startup on the side. In my experience, having a supportive wife, not just someone who will "let" me work 2-3 hours a night, but someone who believes in what I'm trying to do, makes all the difference. There are enough stresses in family life as it is. Add to that the stress of a startup, and without someone who is on board and has bought into it, and willingly bears all the crazy stuff that goes along, I promise it will be a disaster.

Also, the sooner you make money off the start-up, the better. Having something tangible to show for the hours you're away from your S.O. helps the situation tremendously.

For me, I try to stay balanced as much as possible. If I need to take a night or two off of hacking a week to watch a movie with my wife, or take my kids out and stay up past their bedtime, or just grab some beers and do nothing, I do it. You're only hurting yourself (and, by extension, your startup) by burning the candle at both ends, so don't feel bad about taking a night off. I also play it by ear on Saturdays and don't do any work on Sundays.

The toughest thing, I have found, is to be mentally and emotionally "present" with my family when I am not working. Doing a startup is all-consuming. If you give 100% of yourself during your family time, it will make it easier for them to deal with you not being around.