I think even back in the 50s, when I was born, there were a lot of older grandparents. My gran didn't have my mum till she was in her 40s - 1933 -(last born of a large family), as was often the case back then. So even though my mum was only 20 when she had me, my gran was in her 60s when I was born. By the time I was 10, I seemed to be looking after her quite often rather than the other way round.
Things have changed these days though, older people now tend to have more things to enjoy and feel free to do so, rather than just child minding their grandchildren, much as they love them.
I will have had experience of both scenarios I suppose. Was a grandma at 44 by son number 3. And expect I will be via my younger sons eventually (18 at the mo)so will probably be in my 60s for that.
By the way flashy, had the twins when I was 36. yes they were planned before anyone asks.
Well one was lol! By that time older ones were 17 16, 14 & 12 Never been any problem with the age gap.