I was born in the second half of the 1950s. My parents were born in the early 1920s. Both my parents were intelligent, but my mother did almost all of the child rearing, and housekeeping - as was usual in those days in certain communities. My mother would have never left me in the care of a stranger, no matter how short the time period. I don't think that any intelligent person would have ever done that. So to me it is a matter of intelligence, not age.
As a mother and grandmother, the only people who ever look after after my children (apart from me, their dad, and teachers) were their grandmothers, I just couldn't have relaxed if that hadn't been the case.
If anyone had ever approached me in a park etc, and asked me to watch their child for even 5 minutes, unless it was an utter emergency, I would have said:
"No, I'm sorry but I can't take on the responsibility of looking after your child/ren, please either take them with you, or buy them some water from that little cafe".
If I had had my own small children, or my grandchildren with me, I would have added:
"My grand/children would have to take priority over yours if anything happened, so it is a big no"!.