When the walker just won't fit by the baby bed at night....lol.
I think if a person wants a baby and is physically able to bring that child to life then they are not too old.
Do you realize how many grandparents in America are raising grandchildren? Hundreds of thousands if not millions of them. In our grandparents support group there are probably 20 families represented. Some reasons these grandparents end up doing this a second time are their children's drug habits, the death of their child, mental illness, lifestyles, incarceration, etc...
We have one grandmother who is in her 60's and has a whole new family of kids starting with a baby that is just a few months old up to a teenager who is in high school. Her child died of the flu and he had full custody of the kids since the mom had taken off as soon as the baby left the hospital and no one even knew if she was still alive. So this older grandmother who was looking forward to retiring is now having to change diapers, be up at night times again, is now looking forward to working at least 10 more years, etc...she loves these children with her whole heart though and the only thing she would change would be that her son didn't die, that he had recovered and was still there for them.
Another story is a common one. Drug addiction. Some have adult children on the streets, living who knows where, in their cars, with what ever friend will let them crash at their house, who ever wants a piece of hiney for the night, again, not making good choices due to their brains not being in control on their actions, the drugs are in control of their actions. They can get better, get back on their feet, and can become good parents at some point. They might even be able to take their kids at some point and be a parent to them.
All that said just to point out that our soon to be or already there senior citizens are already doing it so if a person even in their 50's wants to have their own child then they should be able to do so.
I don't think a mother in her 40's is too old. It is more common now than anyone really realizes. I know several moms who started 2nd families after a long marriage and then divorce. Marry a 2nd time and they find they'd like to have children together. Their business.
Some find themselves working and working and not taking time then suddenly realize if they don't do it now they will never get the chance so they start up late.
Some have a large family of kids and they love their lifestyle so they keep having more. Until they are done. It's not anyone's business other than theirs. So if a family wants to have a lot of kids then they should be able to do so.
All in all, if a mom wants to get pregnant and has the machinery in working order then she should be able to get pregnant and have that experience.
BTW, my husbands grandmother got pregnant in her 50's or 60's. I know I was shocked that she could still do that. BUT since there was no birth control back then I guess fate or nature decided when people were too old, that was called menopause...lol. When he is doing genealogy he always gets an error message saying he has entered an invalid date for the birth of that child. He always has to go in a click some dots to say it is correct.
She didn't decide she was too old and stop, she thought she was done and found she wasn't. She did have other children but they were all in school or out of school at this time.