The world is aging so fast that most countries are not prepared to support their swelling numbers of elderly people, according to a global study going out Tuesday by the United Nations and an elder rights group. The report ranks the social and economic well-being of elders in 91 countries, with Sweden coming out on top and Afghanistan at the bottom. It reflects what advocates for the old have been warning, with increasing urgency, for years: Nations are simply not working quickly enough to cope with a population graying faster than ever before. By the year 2050, for the first time in history, seniors over the age of 60 will outnumber children under the age of 15. Truong Tien Thao, who runs a small tea shop on the sidewalk near his home in Hanoi, Vietnam, is 65 and acutely aware that he, like millions of others, is plunging into old age without a safety net. He wishes he could retire, but he and his 61-year-old wife depend on the $50 a month they earn from the tea shop.