Clueless baby boomers in New York and San Francisco may joke about the avocado-toast-eating millennials living in their parents' basements — but here in St. Louis, the joke is on them. Here people ages 18 to 35 comprise a surprising 40.2 percent of all metro area homeowners.That's according to a recent study from apartment listing company ADOBO, which rounded up the percentage of millennial homeowners in the nation's 50 largest metro areas.…