I wouldn’t worry about being too old by the time you finish residency – 40 seems old now, but by the time you get there it won’t. I know people who have quit careers and gone back to school to start new careers at age 45-50!
Your other concern, about having a child to take care of, is probably a more valid one. I’ve heard of other mothers who manage to get through it, but at what cost, I don’t know. There’s a thread in the Physician’s forum about all these women who are quitting their jobs, some even leaving medicine altogether. It’s disheartening.