I'm with FF, here. It might depend on what you mean by "examine", too. We can make the assumption that most of the people who work out at the local gym are under 30 just based on what we think about people who work out at gyms without having any experience, then, yes, the assumption is probably something that has never been examined, but, if we have gone to the gym ourselves and tend to see mostly people that look relatively young then the assumption has some basis other than pure guesswork. If you want to call that "examined" then the assumption can be re-examined if we discover that at times we don't normally go to the gym there are a lot of elderly people.