The biggest changes in family life sometimes happen gradually. New employment data suggest one such seismic change is upon us: Job-holding patterns between the generations have turned upside down.

In the past, Grandma and Grandpa tended to retire to a life of leisure in their sixties, while teenagers were expected to work. As recently as 2000, boys ages 16 and 17 were far more likely to hold paying jobs than their grandparents ages 65 to 69.

But...