I think if you've waited until then it can't really have been a priority in life to have a family and it can be a massive change if you've lived a life without kids for so long. Third, fourth and fifth kids are completing a family, not starting one
But why does waiting for a child, for whatever reason, make you too old, when having a subsequent child at the same age isn’t?
Older parents may plan on having fewer kids, so that first child at 36 may be completing the family as well.
People here are citing biology, genetic issues, age related tiredness/less energy etc as reasons for being too old to have first child at 36. All of which still apply if it’s not your first.
I agree with pp, i think it’s more about seeing older women as past it, left on the shelf, dried up and shrivelled old spinsters compared to fertile shaggable young 20 year olds.