I worked three jobs from the age of 15 until I was about 19 and in college .... surprisingly I think all three helped make me successful in life and built essential skills ... I would suggest any job that requires customer interaction, to me the most important skill you can learn at a young age is customer service and how to deal with people from a range of ages, backgrounds and socio-economic statuses. At the end of the day, people are the business of america now and the future.
At the age of 15 I started working at a certain Toasty sub chain, it was a good job, no deep fryers and I worked all three positions, making sammiches, wrapping and cashier. Making sammiches in a fast paced environment and doing it correctly was a challenge, but at the same time I learned how to deal with customers in this environment. I also learned how to tactfully deal with managers, which is an essential skill as well. At one point the time clock broke, and the manager started totaling up the hours manually ... she began shorting me because of an unfamiliarity with simple math and the concept of units. so if the time card would say 4:15, she would pay me 4.15 hours not 4.25 hours ... when you're a kid making $6/hr and you're in honors math classes, you notice these things right away.
My next job was working at a parking lot at a large concert venue. I learned how to stand out in he heat, and how to make friends with coworkers of all ages. This was probably my "adventureland" summer so to speak, lots of fun, lots of drama, saggy boobs at jimmy buffet, hooking up on a security deck, great great fun, working crowd control at crazier shows like ozzfest. This was a boring job, directing traffic, but still to this day I have a good idea of where a car is when its approaching me and if its slowing down or speeding up. You may think you have that down but it pays off big time when you're crossing streets in busy cities and late for work
... I made tons of friends and had tons of fun that summer ...
My next job was by far my best job of my teens ... I served tables at weddings. I learned how to deal with all types, wound up brides, crazy parents, old people, drunk people, little kids, drunker people ... I was pretty good and always polite to everyone. I greeted the owner when I saw him, even though he tended to stay quiet. Eventually I was promoted to a manager and began working both weddings and in the attached country club. All the sudden I was 18 with a manger title, but personally I don't think that would've been possible without my previous jobs.