Monochrome I'm not reading those comments as judgey - and I'm someone who's been trying to become an older mum for 5 years now, so I've got a dog in this race! We all age differently. Some people reach 40 with loads of energy, no aches and pains, etc, while others feel quite knackered with creaky knees. I don't think it's judgey to admit it will probably be harder to run around after a toddler if you have arthritis than if you don't. Also, 40 doesn't seem like the new 20 if you have relatives who died in their 50s; we have to think a lot about the possibility that one or both of us could become terminally ill when a potential child of ours is still a teenager, and how we would provide for them, etc. It affects how we think about antenatal testing and all sorts.
None of this means anyone shouldn't do it. Just that it's worth noting that there will be different challenges, and things it's good to think about and prepare for. Particularly if you're thinking of having a child in a relationship you feel lukewarm about. (Which again is not to say don't do it - but think about what emotional & practical support you'll need and where you will get it if not from partner).
Loads of women have healthy babies in their 40s. I'd love to think I might be one of them. But it's so important to be aware that many, many more who try, don't manage it. You literally only see the success stories: the months of failed fertility treatments, the babies you've lost - those aren't visible to other people. You can't often talk about them without being accused of being negative or macabre (as evidenced by people on this thread dismissing discussion of infertility / higher rates of miscarriage among older women as scaremongering or hand-wringing angst - like it's some sort of myth). It can seem like those in their 40s with happy healthy babies are in the majority. It's just not the case, sadly.
Again. None of this is to say don't try. Just fortify yourself for the possibility that it won't happen, or will involve difficult decisions, or invasive and immensely costly treatments, or loss and disappointment that is hard for a relationship to withstand. It's not a simple choice of leave & have no baby, or stay and have a baby.
Fertility testing is a great idea, but nobody can test for egg quality, which is the main impediment to fertility over 40. Really not meaning to be a downer here, but I'm going through all of this now and I wish that people had been franker with me at the outset about how hard it could be. I wouldn't have chosen any differently, but I would have been more prepared. It's important to hear more than just the stories with happy endings.