Bay seniors please sit down and get a grip. In another shock to our system, Jefferson Starship is celebrating its 40th anniversary this year. Paul Kantner is still leading the group that defined San Francisco rock n roll. Grace Slick is long gone, now an active artist painting, among other themes, the White Rabbit who led many of us on adventures into our consciousness decades ago. If you want to catch the latest incarnation of the Starship, they will be playing Saturday April 28 at the newly resuscitated Sweetwater in Mill Valley, itself 25 years old, handsomely restored with a bar, café and music every weekend.