When we visited my mother's parents, all I took in was that they were old. I thought very little about what that entailed. I was aware that granny had a bad back and some arthritis but that was about it. Most of her life was a complete mystery to me, likewise grandpa's.
Of course one reason I thought so little about old age was that for most people in those days old age didn't last very long. The generally-observed retirement age was 65 and most people only lived for a few years after retiring. Because of that they were less likely to develop the multitude of medical problems that afflict so many of today's oldies. So if I ever thought about old people's health, I assumed they were physically fit rather than frail and decrepit.
I think it's also the case that youngsters, if they think of old age at all, simply don't want to know that one day they might be old and enfeebled, so they try not to think about what's coming down the line. They tell themselves that they'll never get old, or they'll die before they reach old age, or that all the infirmities of old age might happen to others but not themselves.
I was convinced at one time that I wouldn't live very long and would thus avoid old age altogether. In my twenties, I was quite sure I wouldn't reach 30, and was rather surprised when I did. I also thought I might die at 70, as my father did, but that never happened either.