Many people who are the best at what they do started their careers very early. If someone wants do that then I don't think you should discourage them. Being satisfied with your career and work over the span of a lifetime because you spent the time to get good at it is more valuable a few years of goofing off.

Besides, age 13-14 is when you start high school, and that's when things get real anyway. If you intend to apply to a top school you need any advantage you can get, and work experience in jobs that are actual careers is impressive.