Hi, Immabe.
I am in vicinity of your age, with lots of friends having kids (or who had kids in their 20's), so I understand the feeling that "now's the time" or "if not now, when?", or "omg it's almost too late". I'm also in the same boat of being an older non-trad who has had plenty of doubts about whether or not it's a completely ludicrous idea to give up my career to go into debt to become a doctor. So let's break this down into the two issues:
1) Doubts re: medicine - Can't help you. You're on your own to figure out if it's just the kids thing or if there is more at play. You can quit at any time, but it's obviously better if you can figure out if you've been honest with yourself about your motivations and whether or not you really want to go down this path. Everyone has doubts though, so don't let the mere presence of doubt freak you out too much. Just be sure to examine where that doubt is coming from.
2) Kids - Fortunately, you still have plenty of time. Even after 35, plenty of women have healthy pregnancies. And honestly, pregnancy/having kids is going to borderline suck (or at least be moderately inconvenient) at any time in your life. It's not any easier when you're working. Med school is considered by some to be a great time to have kids because there can be some flexibility with watching recorded lectures, etc. Would you rather pass up medical school, get a job in some other field, and try to have kids when you're working full time and have no guaranteed paid maternity leave (assuming you're in the US)?
Personally, if it came down to an ultimatum, I'd vote for pursuing career over children, because there are more avenues for having a family then there are for having a career in medicine. Assuming you don't have any fertility issues, it's pretty much as easy as "have unprotected sex for a few months" and voila. If you struggle with fertility, there are options for IVF and adoption. Not that those are in any way easy, but neither is childbirth (so I hear, anyway). Still, just about anyone can do them. So if you are dead set on sharing your home with immature humans, you can absolutely find a way, with or without a partner, at just about any time in your life. However, if your heart is also really set on being a doctor, there's only one way to do that, and that's to go to medical school.
Lots of people do both and report having very fulfilling lives. Lots of people do neither and also seem to be having a great time. Lots of people really struggle with both kids and career (separately or together). Obviously it's going to come down to your preferences and values, but just know that you still have time and options (for both your career, and your family planning).