Some of our reasons might not apply to everyone, and I'd like to hear about others' reasons (for and against) as well. I plan to add this entry to my memories and point prospective parents to it.
Academic timelines
In academia, there's really only two times you're not on the clock: graduate school and post-tenure pre-department-chair (well, and also retirement, but I'm not planning to be ovulating then). In a graduate program, it's ok to take an extra semester or year to finish a degree. Employers don't care. In contrast, taking some time off before getting a tenure track job means it's harder to get back into the workforce. If you have a tenure-track job, some schools will let you extend your tenure clock for childcare; some won't. You're not guaranteed of getting a school which agrees on paper, let alone in reality.
Health
We're left with two real options: grad school, and tenure. I'm 25 in grad school, but I don't expect tenure before I'm 40. It is easier, cheaper, and less complicated to have a baby at 25 than it is at 40. It is also easier to keep odd baby-centric hours at 25, easier to bounce back after childbirth, and easier to keep up with a toddler. Plus, having a baby at 25 instead of 40 means 15 more years of grandparenthood for my folks, and a chance for Lea to meet her great-grandmothers.
Living on the cheap
It's true that tenured faculty make more money than graduate students. However, as a graduate student with a 411-operator husband (he makes as little as I do), I make enough money to buy a house, have two cars, and go on dates every week. We have fantastic health insurance, but if we didn't, we would qualify for state health insurance. We make enough money to have a kid now. Breast milk is free; cloth diapers are cheaper than disposables. Everyone gives you infant clothes. Small babies are cheap. If I were to have kids at 40, I'd be worrying about how to fund their college tuition and my retirement at the same time; this way, I can spread out the big expenditures.
Daily life
As a graduate student who's not taking classes, I'm only scheduled to teach and have meetings. As a senior graduate student, I get first pick as to which classes I teach, so I can arrange my schedule so that Lea doesn't need regular daycare and I can keep working full-time. My advisor is extremely supportive and lets me research at my own pace and keep my own hours as long as the work gets done. My peers are having babies at around the same time I am, so we can have playgroups and daycare swaps together, and that gets me adult social interaction as well as cheap babycare.
