SAN FRANCISCO — One of two skydivers who died plummeting into a Northern California vineyard was 18 and jumping for the first time, and his family was there watching when he hit the ground, the owner of the skydiving center and local authorities said Sunday. San Joaquin County sheriff’s officials Sunday had not publicly identified the new skydiver or the skydiving instructor killed Saturday in Lodi, 85 miles east of San Francisco. One of the two was from another country, said Bill Dause, owner of the Lodi Parachute Center, where the accident happened. The young man jumping for the first time had a brother and several other people with him at the skydive center.