I think when you feel like you've done all the things you want to do before you have a child and your life completely changes (and doing those things becomes a lot more difficult) and when it won't make doing the things you want to do in the next few years too stressful or overwhelming to manage.
My husband and I had our first when I was 32 which was a good age and time for us. We'd spent our 20s doing whatever we wanted, enjoyed life, traveled, lived and worked overseas, had lots of time to be selfish and do all the things you can't easily do with small children. We didn't own a house and actually we still don't, which is more of a personal choice as we wanted to save and wait until we were further along in our careers so we could buy the sort of house we really wanted, so renting made a lot more sense. But we were sort of at a place where we were ready to close one chapter and start another and not have any regrets about all the things we were going to be leaving behind. I was still half way through a PhD program when our daughter was born, but that was actually a blessing because it meant I had the time and flexibility to be at home and to adjust my schedule around her. The downside to studying while you have a small child is that childcare is very expensive and a PhD doesn't pay much, so it's been a financially tight few years, but manageable. I know some people can manage working at home and a baby, but frankly, I am not one of those people. A baby is all-consuming. There is rarely a free moment in the day, even when they nap, you're either getting caught up on the mountain of stuff you need to do that you can't get done while they're awake, or you need to sleep yourself because you only got three hours the night before. And evenings are usually short and interrupted, I would put my daughter to bed at like 7, she might wake up twice in the evening and then we'd go to bed ourselves at 9:30-10. You can't get much done then either. And childcare is about £800 a month. It's more than our rent! So you really just need to know that you are at a place in life when you can clear your schedule and have time and money for that sort of commitment. I'm glad I had my daughter while I was still in school, because otherwise we'd only be looking to have our first now and I'm 36. I wouldn't have wanted to wait that long, but it does take a lot of family support and money to make it happen.
We are looking to have our second soon and though we're more financially secure and settled now, life is definitely more stressful and I have a lot more professional pressures on me than I did when I was younger. I think you're ready when you're ready for your life to change and to give up some of the comforts and the easiness and flexibility you have now and be okay with that. Having a baby is a major life change and you'll never get back what you had before, even though you'll have a new kind of life. My husband and I used to have nights alone and dinners out and weekend trips away. We have literally spent 4 nights alone in nearly 4 years. Otherwise, we're home, in bed early most nights and up by 6am every day, 7 days a week. If you feel ready for that, you're likely ready. I think it has much less to do with financial security than it has to do with if you're in the place in life where you can genuinely cope with it and come out stronger on the other side. That's not to say being a parent isn't amazing. It's the best thing I've ever done in my life, but it's also so, so hard and so exhausting.