Millennials are struggling to find affordable housing in the U.S. At the same time, many baby-boomers are retiring and they would like to stay in their homes that have lots of empty bedrooms. Real estate site Trulia sees a match made in heaven. The site has calculated that there are 3.6 million bedrooms vacant in the 100 largest US metro areas. Trulia estimates that homeowners can make on average $11,000 a year renting out rooms renting rooms while millennials can save a lot of money renting from boomers rather than getting their own one-bedroom apartments in those cities.