With the high cost of tuition, a lot of families start thinking early about how to save enough money for their children’s college education. One option for people who have the means to do that is a college savings plan known as a 529. It lets people invest college savings without taxing the gains on that investment. But a report from the research firm Morningstar says only about 16 percent of families who are saving for college use the 529 plans, sponsored by states. But why might that be?