I guess we'd have to determine who was the youngest cast or crew member in the OT. In Return of the Jedi, there is a small child being comforted by an Ewok. My guess is he was born in 1978-1981. I'm not sure if any Ewoks were very young. If we count that kid, my guess is 2050 will be the last year Disney can parade a member of the original cast/crew. Of course Lucas could outlive them all.