It seems to me that a long long time ago, life spans should have been short enough that you would die soon after giving birth to your child, and not live long enough to see their grandchild. Is there a record of who the first grandparent was then? How about great-grandparent?
Consider the following:
Average lifespans in the past are dragged down by infant mortality, not by people dying en masse before they reach fifty;
hunter-gatherer societies are filled with grandparents, so it's not unlikely that many prehistoric humans would have known their grandparents;
Many if not most human societies - including those without written records - have a place in culture, in family units, and in child-rearing for grandparents. It's even speculated that menopause developed to strengthen grandparents' involvement in child-rearing.
if you give birth at age twenty and that child gives birth at age twenty, you're a grandparent at age forty. In a society with no birth control, young marriage, or youthful sexual activity (or all three), people can be grandparents at aged 30.
In short, we don't know who the first grandparent was, because the question is built on a platform of false assumptions. Heck, even the first great-grandparent could've been under fifty years old.