Minnesota is headed toward a significant milestone in which more of its residents are senior citizens than school-aged. A state demographer told a panel of top legislators Monday that Minnesota is on course to reach that point in 2020. It represents the point when those older than 65 years are a bigger share of the state population than those between 5 and 17 years old. The demographic trends are important because they suggest where future budget pressures lie. Medical and long-term care costs the state subsidizes are already rising fast, crowding out available money for other things. And more retirees mean fewer income tax dollars will be coming in.