If you're just talking about old enough to have a grandchild, 50 is plenty old -- indeed, old enough to have gotten married after college, had a kid who got married after college, and that kid now has a kid of their own. At least one of my high school classmates did that. (I'm 51.)

If you're talking old enough to be, say, a college student's grandma, then obviously your 70-90 estimate is much more likely.

(Bigger picture: I've been professionally programming for three-and-a-half decades. I find myself more and more annoyed by the user-unfriendliness of tech as I age. As an on-topic example, earlier this month Amazon's mail-to-Kindle feature rejected an EPUB I sent it, and when I used Calibre to convert it to MOBI and sent that instead, lectured me that MOBI was obsolete. I guess this is especially annoying because it seems like they're replacing a service which basically never failed with one that is failing like 10% of the time I try to use it...)

> Grandmas are 70-90, not 50.

To a reasonable approximation:

  - Grandmothers of infants are between the ages of 40 and 80
  - Grandmothers of HN commenters are between the ages of 55 and (dead for decades)

> I work with many folks around 50 and most of them are functionally technologically illiterate

Most people of all ages are functionally technologically illiterate.

Many people around 50 are functionally literate in different technologies than you are. Those may be less relevant, or merely invisible to you as a user.

The most common child bearing years are 18-25 ergo most people become grandparents between 36 and 50 while only 43% will live into their 70s

My son's grandparents are 55-63, and his one great grandparent is 76