Answer
$S = \{N, N = all\ natural\ numbers\}$
Work Step by Step
The sample space of a random phenomenon is the set of all possible outcomes. While it may seem reasonable to say that the range of age possibilities is 18-22, there will be outliers, where students attend college at a younger age, or at a much older age. To account for these outliers, it is safer to assume that there is a possibility that someone of any age could be in the class.
