The problem is, how do you define smart? I can blow away IQ tests, but that's not much use when trying to restart a subsistence level farming economy whilst simultaneously retaining scientific knowledge. That said, I'd make a cut-off of maybe 30 except for really important, highly technical skill sets that just aren't available in younger people, in order to get a longer range of vitality and reproduction whilst maintaining a reasonable maturity and skill set. Weed out any known genetic problems or chronic illnesses, weight it by standardized IQ and a health index, and select maybe 50% of the population from the intersection of the top of that matrix and a matrix of critical skills. Then I'd select maybe 25% for selected individuals (with families) in highly technical skill sets, 10% just additional, extra-healthy, extra-bright young people (18 - 25, always need labor and breeders), and 15% randomly selected from around the world with a high health index and a modest IQ, weighted heavily toward black and Aborigines. (Blacks and Aborigines, including the white Ainu of Japan, have older lineages and therefore higher individual genetic diversity than whites, Asians, or Latinos, and genetic diversity is crucial to small-group survival. I'd want as wide a range of genetic phenotypes as possible, but weighted toward those groups with the most individual genetic diversity.)
This sucks, actually. I can't think of any criteria that would allow me to select myself. (Unless of course it's MY shelter.)