Excellent point. But that still doesn't leave many grandmothers around (1 in 3 at best?). And they didn't have many years beyond their reproductive age.
Um, think about your numbers a bit more. 1 in 3 what? If 1/3 of the population was over 30 and the average lifespan of people living past infancy was 48, there were probably a lot of grandmothers around. Especially if women were having their first children in their late teens, which is just a guess but seems likely.