Have a kid in your early twenties and they have a kid in their early twenties, presto, you are a grandparent and you have not even hit 50.
And that is a good age to have them because you still have a bit of energy left enjoy them.
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.