Exactly this. There are so many posters who insist that everyone they know started having children in their 40s - despite the fact the average age for a woman to have a baby in this country is 30 to 31.
I live in an 'upper middle class' village and I know women from all walks of life ... Many here are semi professional, professional, top of the tree in industry and commerce and banking and medicine. I know women who are.... vicars, dentists, doctors, paediatricians, consultants, lawyers, teachers/head-teachers, you name it. And all of them had their first child before she was 35. Every single one.
Some of them had the first baby AND the last one before 35. I think I can count on the fingers of one hand, the amount of women I have known in my life (nearly 60 years) who have had a baby over 44/45. and let's just say in all the cases, it didn't really end well. I won't say any more than that.
But yeah as you say; on Mumsnet, some posters don't know anyone under 5 foot 8, all their children are tall and athletic, (and gorgeous and gifted,) everybody they know (including them) are thin - like a size 6. Also, everyone they know is on over £100,000 a year, and their DH earns WAAAAAY more, and everyone's got 50 or 60 thousand pounds in their smallest savings pot, and a pension with £850,000 in it. They also ALL have a cleaner and often have a gardener. And they act all faux shocked that everybody isn't the same. It's a parallel universe!