@ColdTattyWaitingForSummer
I do wonder if some of these children born a few generations back to much older mums were actually grandchildren born to unmarried teenagers and covered up.
This absolutely 100% DID used to happen. Indeed I know at LEAST half a dozen families where this happened. There was a 15-19 y,o. single daughter, who would buggar off to a camp or a trip away for 5-6 months, and amazingly, their mother (usually mid 40s to 52-ish) would have a baby at almost the SAME TIME. 
Lots of people guessed it, but very few people would say. Some people kept it secret for many years, indefinitely even.
As @NottonightJosepheen said, there were many girls back in the past who discovered their sister was their mother.. (when they were maybe in their 20s or 30s...) and their actual 'mother' was in their mid to late 40s.
Even the people on here who say that is not what happened with THEIR family. They can NEVER know that for sure! 
A few will be in denial, like that would NEVER happen in their family!
Closet, meet the skeleton! 💀
I know very few women who have become mothers (genuinely) past the age of 45-46, maybe only 4 or 5 in my lifetime. But of the ones I HAVE known, there have been issues with the baby/child, from childhood right through to adulthood. There are sooooo many risks. No WAY would I do it. Nope.