Back in the 80's I used to have a secretary (yes, that's what they were called back then!) who was a grandmother in her 30's and a great-grandma in her 50's. Her Mom and Grandma were still alive so that meant the eldest was a Great, great, great Grandmother! (She was in her 90's and the whole family got featured in the newspaper once - having six generations alive at the same time is pretty rare!)
Around 2005, while I was the Director of a non-profit organization, one of the others on the Board became a grandmother at 39. I guess it just runs in some families!