It was made in the mid 1970s. Space ship was being sent to colonize some new planet. It was going to take a long time (30 years?) to get there, so the ship was crewed with a couple of adults, and the rest were all 15 or so. That way, there would be a few old people, and a bunch of 40-45yo to start the colony. Anyone? (NO, it was not the original Battle Star Galactica.)
