Mine were 54 and 58 I think (sibling had first grandchild).
I had one living grandparent when my dc were born, she was 94 when her first great grandchild was born and 65 when she became a grandparent for the first time.
Ironically the only one of my grandparents to meet her great grandchildren had her first child at 36, positively ancient in the 1940s... My grandparents on the other side were a generation younger but died decades younger sadly.