There are no hard and fast rules - just do what makes you happy. If having a family will make you happy, just do it! That's what I did, I'm 27 with 2 kids (and struggling to find time for my side projects) and it works great for me. If you want a wealthy family, then you're better off waiting, because not only do kids cost (primarily for the bigger house/apt in neighborhood with schools), they take time away from working longer to make more $$. But the right answer depends on you.

But if the goal is to have successful startups, then live cheap, start young, and try and many times as you can endure! A 40 year old who has already done 10 startups is probably a lot more likely to succeed than one who hasn't done any.