I'm of the age now where I have to start actually thinking about how to balance career and family and all that, so I found this mathmatical model of the benefits of having kids as a function of age kinda fascinating. From that link you can download an excel file and instructions on how to fill it out. It has a bunch of curves, representing your professional productivity and social life and "ability to enjoy a child" as a function of age, and takes into the relative importance of family, career, and social life, and the different options -- taking varying amounts of time off or working part time while the child is young. I find it funny that it seems to imply that the "utility" of having a teenager is relatively low. It's supposed to automatically incorporate fertility and medical issues as well. It's aimed at women, but really it seems like the best idea would be for a couple to use it together.
It's a pretty impressive model, I'd have to say, although it can't quite incorporate every factor that comes into play in a real decision -- relationships with your boss and co-workers, how close you live to family, how many kids you eventually want to have, financial considerations... Trying to include everything might make the model unworkably complicated, of course. It is nice that it doesn't just tell you an "optimal age" -- it gives you a graph of "utility" as a function of age.
There's a press release about it that anyone can access, although it doesn't tell you much, and a journal article about the model that you can read here if you have access to academic journals.
I know some people would think this is a ridiculously inappropriate problem to try to solve mathematically, and part of me agrees and finds it a little bit hilarious. But really it's just a more sophisticated version of the old "pros" and "cons" list. If you're comfortable with mathematical modeling at all, it's pretty easy to use. The questions it asks are basically "If you think your professional output will drop when you have kids, how much do you think it will drop? How long will it take to go back up? Will it go all the way back up, or if not, to what fraction?" and likewise for social life, and a few other factors. It's still oversimplified, but it's a nice tool to have, to help you keep all of the effects, over at least a couple of decades, of this kind of big decision in mind all at once.