I had fertility tests done in my mid 30s and was told I had a good egg reserve and could probably get pregnant straight away (I didn't want to) but was also told by the consultant that I was very fertile, so should be careful if I didn't want to fall pregnant.
I had a termination (unplanned pregnancy) at 17 and a miscarriage at 21. I think I also had another miscarriage at 31, but I wasn't sure if it was one or not, I had had two very positive pregnancy tests though and then had a very heavy what I thought was a period, and then next time I tested, it was negative. My big thing was, after having the termination, apart from when I was pregnant (the 31 year old one wasn't planned, 21 year old one was), I was desperate not to fall pregnant, so despite being on the pill, I panicked about it and when I came off the pill at 30 and tended to use withdrawal method (yeah I know, unreliable) I got the MAP a few times for protection.
A colleague of mine was married and then divorced and then remarried again, her DM is a gynaecologist and told her that apparently the cut off point for pregnancy was 37, so she waited until then to start trying, got pregnant but then had her second child a year after the first one. Other women I've known, one got pregnant at 35 and she didn't think she could get pregnant before (when she tried it hadn't worked with a previous partner), another got pregnant at 40/41 but had a miscarriage but then 3-4 years later fell pregnant, a few friends have got pregnant late 30s/early to mid 40s.
Older celebrities, almost all of them I'm guessing have had expensive IVF and/or donor eggs/surrogates.
I think to a certain degree women are misled into believing they can have children any time they want. A schoolfriend of mine, we were all mid 40s and met up for a school (Catholic convent school) reunion, one of them stated at the table she thought she was ok to get pregnant then and we all told her kindly we thought she'd missed the boat, she has an older and a younger sister and her close friend is a midwife so you'd think she'd know from speaking to them. My brother's ex-GF from a few years ago, I met up with her a year ago, she's 47/48 and told me "if I want another baby I can have one if I like" - she's having regular periods, but there's no guarantee of course of it happening nor any issues (she decided not to go ahead). I've always known I could if I wanted to have kids but was very much half and half about it and would have had to be with the right man, not just any old man, due to my DM being a single parent.