I'm seventeen & Ive been working since i was sixteen. I'm assuming that by underage you mean under eighteen? Some kids need the money or need to help their families, or some like me want experience & something to put on resumes & college applications. Its also really helpful if you need a good job, its better to have a reference than no experience. Ive worked at customer service & as supervisor before i turned seventeen, when i turn eighteen i can apply for a manager position. Having a job is a good thing.
In the US, it's 14 with a work permit, however you can work earlier than that for family run businesses, depending on state laws, and even be paid less than minimum wage :) I started working at 11 years old, during the summer.
In Maldives, they have very little enforcement of their existing laws...so I wouldn't be surprised if people even younger were "working" here.
I'm in New Zealand and I started waitressing when I was 15, it's just different rules for different countries.