Middle School Age: Many middle school kids are mature enough to start working. Volunteer programs give back to the community while exposing youngsters to the rigors and responsibilities of a real job. Paid jobs include babysitting, yard work, dog walking, car washing, tutoring, or crafting and selling their own goods. The article, Young teen's dilemma: too old for camp, too young to work, shares some good ideas.
High School Age: These kids have reached an age where they can start holding down real jobs in offices, retail stores, restaurants, etc. Many businesses offer internships to high school students. Colleges hold summer programs, where they can investigate majors and maybe even earn college credit. And, with thanks to Nicole Dieker, here are 100 Fun and Lucrative Summer Jobs for Teens.
Here's a promising site for teen job seekers: