I started babysitting when I was 11, for neighborhood families and my younger cousins. I delivered two different newspapers and in high school, I babysat for various people 4-5 times a week (I had 15 families who called me). During the summers, I nannied full time (everyday, all day, bleh). In college I was a computer technician, a TA, and lab assistant. Combined, I worked about 20-30 hrs a week while school was on and then did the technician job full time (40+) over breaks and summer break.
Why did I start working? Because I wanted to go to movies, buy clothes, go out for dinners w/ friends and such and my parents would laugh in my face if I asked for spending money. In college, I worked to pay for books, food, rent, and utilities. In high school, I worked more in the summers for something to do, since everyone else worked too.