Men? 90.
Women? 40 (and that’s pushing it).
Men do not carry the burden of having to take care of children. That usually falls to the mother (despite all efforts to change it in society). Yes, there are a lot of househusbands now, but they are still in the minority. So, men, basically can continue to father children (like Charlie Chaplin did) until they keel over. They won’t be changing nappies or driving kids to school. The mothers will.
I say 40 for women, because honestly, exhaustion sets in. Hormonally and physically, your body begins to change in your forties. My grandmother had my father in her 40’s and she stayed home to care for him, and she lived to be in her nineties but she had her daughters and an extended family to help her out.
So, 40 (give or take a few years) is a good age to stop. However, there will be exceptions to the rule…..now that IVF is allowed on the NHS you can have children into your 60’s! If a woman decides to do that, she takes her health and her well-being into her own hands and she may well end up raising the children alone. There are a lot more hazards, I think. That being said, if you have unlimited amounts of dosh, family support, help at home, a great relationship and all the ducks in place…a woman can keep having babies as long as she wants. But she will have to still tax her body to do it. And you have to bear in mind, that your children may be left without parents. That would be devastating.