(St. Paul, MN)  —  A decrease in youth tobacco use in Minnesota cities is attributed to a move setting the age of purchase to 21.  A study conducted by the University of Minnesota and Minnesota Department of Health says the minimum age requirement brought about lower levels of tobacco use by eighth- and ninth-graders.  About five years ago, some cities and counties raised the tobacco purchase age from 18 to 21 at a time when vaping was on the rise.