It depends on the location of the research institution and the communities they’ve trying to recruit from. For example, when I worked in research, we were based at a VA hospital near but not in a large city. We didn't go into the city to recruit study participants because we were too far away for people to be willing to come. We recruited young people from the local community college, and senior citizens from the places you'd expect to find older folks: the senior center, churches, restaurants that had 4pm dinner specials, etc.