Have a child at 16, who has a child at 16, who has a child at 16, and you can be a great-grandmother and not yet be 50.
And my great-grandmother was married at 16, so at one time this would not have been considered odd.
I am simply commenting on the math of being a grandmother at fifty. If our oldest waits as long as we did to have children we will probably be dead before becoming grandparents.
In a perfect world I would be a grandmother now but a perfect world does not exist so you work with what you have.