The reason to start a startup when you're young and single is because you have no commitments. Once you have kids, you can't afford to take that kind of chance with your life because you're also responsible for their lives. But a spouse/significant other who's genuinely supportive is like a partner in the company who's sharing your equity -- if you make it big, he or she makes it big too.

On the other hand, a spouse or significant other who's not supportive -- or who means to be supportive but who doesn't actually offer much support, which is worse -- can put enough of a drag on things to make the company fail.

And as brlewis notes, a lot depends on the pace you're trying to run the company at. If you're using the traditional VC-funded web startup model and working 100-hour weeks in the hacking zone, your relationship with your spouse/SO is going to suffer. If you're using the consulting/micro-ISV model and expanding the business gradually, it's likely to put a lot less strain on the relationship.