[Edit : It turns out people have said the same thing while I was looking for the right source to confirm my point, so I guess this comment's a bit redundant now. Still leaving it in case someone's interested]

The number's correct but...

Child mortality The most significant difference between historical mortality rates and modern figures is that child and infant mortality was so high in pre-industrial times; before the introduction of vaccination, water treatment, and other medical knowledge or technologies, women would have around seven children throughout their lifetime, but around half of these would not make it to adulthood. Accurate, historical figures for infant mortality are difficult to ascertain, as it was so prevalent, it took place in the home, and was rarely recorded in censuses; however, figures from this source suggest that the rate was around 300 deaths per 1,000 live births in some years, meaning that almost one in three infants did not make it to their first birthday in certain periods. For those who survived to adolescence, they could expect to live into their forties or fifties on average.

So reaching 50 wasn't too rare for someone who had survived childhood, and given how people often started having children younger then, that was well enough to be grandparent. Doesn't mean everyone would've gotten to known their grandparents, but it wouldn't have been super rare either.