Have a kid already, could, under the right circumstances, be convinced to do it again😛
Here are the variables:
1) pregnancy could be difficult for you (I'd put that on the less likely end of the spectrum if you're under 30, but whatever, it could happen)
2) baby could be a difficult baby (either behavior or illness)
3) baby could become high maintenance child (see 2).
For me, it was mostly 1 and 2. I had horrendous all day AM sickness, and I can't imagine trying to study at med school level, or be around anything smelly, under those circumstances. Baby was awesome, but a lousy sleeper, and once again, I think that would be tough to take when you are professionally under duress. Now he's a piece of cake (4 is a great year).
For myself, I would want the easiest schedule possible for the first 2 years of baby's life, and to not need to think too hard during the pregnancy (all those hormones, the increased blood flow, the throwing up, they made me stupid).
I think I'd probably schedule the easier clerkships at the end of 3rd year, try to get pregnant 3rd year, and then take off a year after 4th year to do research or something else cushy, then start intern year one year late, when baby is at least 18 months old. The couple of months before intern year, I'd want hubby/mom/mother-in-law/all-of-the-above very available, or for junior to start day care then, so that his/her schedule was very established when I actually started. That support system stays in place through intern year, de-escalating as necessary.
Or, you just wait till intern year is complete, and choose a residency where the PGY2+ aren't crazy hours.
I don't think 30 is too old. I do believe age affects parenting style, for better or for worse, but so do lots of other factors, like your upbringing. Most 30+ parents I know tend to have a harder time saying "no" to their children, and fuss over them more than I would, but then, a lot of the under-30 set I know have a hard time paying the bills and being patient w/ their children (myself included there). Maybe that's maturity, but it could also be that those 30+ were so terrified they'd never have a child, that said child is up on a higher pedestal.