At the moment, I'm 25, single, and childless. While some may regard these facts as a bit depressing (because I haven't found that special someone), it has some advantages. Most notably, I have more personal freedom and fewer non-professional obligations to meet. Because professional development during graduate school is so important to landing a job in philosophy (especially since the 2008 economic collapse), being able to focus more narrowly on philosophy is a benefit that I'm hesitant to minimize.
Having said this, the majority of graduate students I know are not in my situation. Most of the ones I know are older and married. Some of them have a child, and a few have more than one. I have seen plenty of examples of graduate students who have been able to manage their marriage during graduate study without any noticeable difficulty, so I infer that having a spouse does not meaningfully affect one's chances of having a prosperous graduate career. (In fact, having a spouse to help ease the stress of graduate study probably offsets whatever so-called "inconveniences" to your professional life arise.) But the same cannot be said for having children.
I have known several graduate students who had children midway through their graduate career. In every case, the same pattern transpired. Following their child's birth, they were almost completely absent from departmental events for a while. They rarely attended their course meetings, never attended special events (including those that featured distinguished visiting speakers), and were never spotted around the graduate student offices. When they eventually snapped out of this phase (usually after 1-2 months), their involvement in the philosophical community was still significantly reduced from where it had been when they were childless.
It's not hard to imagine why this is the case. Having a child involves undertaking responsibilities that have no equal: you are fully responsible for the development and welfare of another human life. The burdens of care impose stress on you financially, physically, and emotionally in ways that nothing else can. And unfortunately, in the tug-of-war between the needs of your child and fulfilling your obligations as a graduate student, your child carries more weight most of the time. Moreover, observing your child's first steps, hearing her first words, playing with her in the evenings—these activities tend to be so much more rewarding than, say, reading that article for tomorrow's graduate seminar, revising that paper draft to send off to a conference, or working on the ever-so-daunting dissertation. The temptation to let yourself underachieve grows (as does the temptation to accept it or rationalize it).
The strain created by having a child is also not likely to disappear when you finally finish the dissertation. Given the difficulty of finding tenure-track employment right after graduate school, it's likely that you, your spouse, and your child will be bouncing around to temporary positions for a little while. During that time, it will be even more important for you to publish and build up your CV so that you can acquire a tenure-track position, but the demands of your child (or children) will still be present. Moreover, as your child gets older, the impacts of your moving (which may be frequent) could impact her more significantly. (They won't remember much of the move at 3 years old or younger, but once they're 5 or 6 and start to make friends, the impact of moving could be quite significant to them.)
I do know a few professors who are able to juggle their professional responsibilities with their personal commitments to multiple children; some of them were even able to do this during graduate school. If I were like them, I would offer some advice on how to balance these commitments successfully. But since I lack that experience, I'll offer a more candid, less compromising sentiment: wait until you have finished your graduate education to have children. Ideally, you'd wait until you have tenure (so that your career is firmly established), but because you can't achieve this until many years after graduate school, such a standard seems implausible.
Of course, one may wonder how I, given my autobiographical notes above, can make such a bold statement about how people ought to conduct their personal lives. After all, how can I understand the importance of one's decision to have a child when I have never been in a position to seriously consider the decision myself? I may not have a convincing answer to someone who presses this objection. I have only my personal observations and some anecdotes from graduate students and professors who have children. It's become clear to me from these anecdotes that graduate students who have children, even if they accurately appraise how valuable their child will be to them, usually underestimate just how great the sacrifice will be and how much it will (professionally) cost them.
I also confess that I am not particularly sympathetic to those who feel they must have a child before the age of 30. (Perhaps I'm biased because I was born when both my parents were in their mid-30s.) Mainly, I fail to see what great loss it would be to wait a few years (hopefully until after the dissertation is complete) when the professional benefits appear so signficant. Moreover, the child is likely to benefit because you will be in better position to secure a tenure-track job, leading to higher income and a stable location. For graduate students in their mid-30s, I admit that their window for having a child is narrowing, and this fact can weigh very heavily on them. Some, I suspect, do not envision that they could live a genuinely happy life without having a child. This dreadful dilemma, where a person faces such a basic and significant conflict between personal life and professional life, is one with no easy solution and is best avoided altogether. This highlights one significant reason why it is beneficial to begin graduate work in philosophy (and likely in any field) shortly after obtaining her undergraduate degree.
I doubt that this post will persuade all (or even most) philosophy graduate students from having children before they finish their dissertation, but I hope it at least makes them realize the professional costs of their decision to do so. The journey to a tenure-track job in philosophy traces a long uphill path. I advise any philosophy grad student to be cautious about making that incline any steeper than it already is.