Lots of young EAs are struggling with the issue of whether, when, where, and how to have kids, and whether becoming a parent will undermine being an Effective Altruist, in terms of opportunities costs such as career, time, energy, money, focus, and values.
For whatever it's worth, I'm happy to answer any questions you might have about parenting -- its pros and cons, ethics, practicalities, etc.
Background: I'm a 57-year-old dad; I've raised a 26-year-old daughter and a 6-month-old baby. I've also helped raise a teenage step-son, and I come from a big, close-knit family (I have about 30 cousins.) I've lived as a parent in the US (mostly), UK, and Australia. I'm also a psychology professor who's taught courses on parenting-relevant topics such as behavior genetics, educational psychology, evolutionary psychology, human intelligence, evolutionary game theory, and decision making. I've been involved in EA for the last 6 years, and I have a pronatalist orientation, with an interest in population ethics, reproductive bioethics, gamete donation, and cognitive and moral enhancement. I'm not an expert on every practical or scientific issue about parenting, but maybe my perspective could be useful to some EAs.
purplefern -- I'll write separate replies for each of your questions, so any further comments by others on my comments can be fairly well-focused.
Regarding optimal age and timing, and career/health tradeoffs:
For a woman, the main age concerns regarding health, I think, are (1) likelihood of being able to get pregnant declines fairly strongly during the 30s (but there are very big individual differences in this rate), (2) likelihood of baby having genetic defects (eg Down syndrome) increases fairly quickly in the late 30s and 40s (but implications of this depend heavily on whether a woman is willing to use genetic screening and abortion as quality control.)
For issue (1), I think it makes sense for women to get their AMH (anti-Mullerian hormone) levels checked regularly from their late 20s onwards. This is absolutely crucial in predicting how much longer one's likely to remain fertile -- it's a test of 'ovarian reserve'. AMH seems to be a stronger predictor of a woman's remaining fertility than her chronological age is. (In many countries, you can get an over-the-counter on in-lab AMH test for about $100-200 that just requires a finger prick or blood draw.) It's also very helpful to know when one's mother, older sisters, or female relatives reached menopause. To risk-hedge, it can be helpful for women to freeze some eggs and/or embryos by age 30-35, which could use later by the woman herself or by a surrogate.
Likewise, for men, I think it can make sense to get sperm checked with a lab semen analysis (typically less than $200), to assess semen volume, and sperm count, vitality, motility, and morphology.
It baffles me that many smart young men and women who invest hundreds of hours into planning their work career won't spend a few hours getting the crucial tests that would allow much more accurate planning of their reproductive career.
Overall, I think the optimal time to start having kids depends much more on one's romantic partnership situation than on one's education/career. If you've found an excellent mate who's compatible, committed, reliable, pro-child, and likely to be at last moderately successful and financially stable, then the best time might be right after marriage, whenever that is. (Having a kid with someone, without the many legal protections of formal marriage -- protections which seem silly and outdated until the point when you really, really need them -- is foolish and often regretted, IMHO.)
I'm most familiar with the academic situation when smart young women are trying to decide whether to have kids in grad school, or after they get tenure -- they assume that the 6 years of intense tenure-track work as an assistant professor will make pregnancy impossible then. I think that is a very misguided way to think about it, for a few reasons (1) most universities are actually very generous with parental leave for faculty, and you can pause the 'tenure clock' multiple times before going up for tenure, (2) professors aren't actually that much less busy after tenure than before, (3) realistically, in the current job market, most people who get PhDs in most fields will never get a tenure-track job, so there's a huge danger that women get stuck in 'post-doc limbo' for several years in their late 20s and early 30s, then don't get a tenure-track job until their early/mid 30s, then don't get tenure until their early 40s... and then it might be too late to have kids. There might be analogous issues in other fields such as medicine, law, finance, etc.