Old hands are hard to replace  Experienced workers are in demand
in the nonprofit sector, too.   Bobbie Driscoll, left, and Judy
Vennewitz greeted regulars in the lunch serving line at the
Dorothy Day Center, where they have volunteered for 22 years.
Photo: Bruce Bisping, Star Tribune  CameraStar Tribune photo
galleries  Cameraview larger  For 22 years, Joe and Bobbie
Driscoll have cooked warm meals every week for needy folks at
the Dorothy Day Center in St. Paul. When they walked out the
door after retiring Wednesday, it was unlikely the center would
find anyone like them again.  Nonprofit leaders call them "the
greatest generation of volunteers." Largely retirees who often
worked one job for life, they've brought the same dedication to
unpaid service. Now, while more than 1.5 million Minnesotans
volunteer, one of the largest numbers in the nation, few are
sticking with an organization more than a few years.  Their
attrition is prompting many charities, arts groups and hospitals
to recast their volunteer pr