Note that the manager offers only one premise in support of his argument: Several recent promotions have been given to people older than Sullivan.
Do these promotions prove that age was not the deciding factor in Sullivan's case? An important thing to pay attention to in the stimulus is relative language like "older." It is no accident that the stimulus describes Sullivan as "older than the competition" rather than simply "old." The issue at hand is Sullivan's age relevant to the competition. So, when we are told that the people older than Sullivan did not have younger competition, it shows that this is a poor comparison to Sullivan's situation.