Generally speaking, American youth have a poor work ethic. There's an unwillingness to work nights, weekends and holidays, or on a sunny beach day. If the work is too tedious, they'll leave after a few weeks or months, or do as little as necessary to get by without being fired. Employers also know that teenage employees are not as dependent on the salary as an older employee, so they may not take the job as seriously.
I see it in EMS where the young employees complain about the late jobs, the schedule, lack of meal breaks, the monotony, etc, and they have three minutes on the job, their first job at that.
Basically, if the employer chooses to hire olde remployees, they hope to get a more stable worker who will do their job more effectively, and better appreciate the opportunity. This is a generalization, of course, but this wouldn't be a common practice if it wasn't true enough of the time.