Babysat from about 12 or 13 on, and then at 16 I had my first paycheck-type job: camp counsellor at a sleepaway camp. Where I lived, with one day off per week, for a little more than 2 months.
And I got paid 300$ for the entire summer. I don’t even want to work that out hourly.
I was babysitting from around the age of 15. I got my first “proper” job at 16 or 17, working in Burger King.
From 12 to 16 I had a paper route. Actually a double route because I took over my brother’s route and another one that opened up. At 16 I began working at Consumer Distributors, a catalog warehouse store that I believe went belly up a while ago.
When I was 14 I started working as a softball umpire. If that doesn’t count, then it was the programming job I got when I was 18.
I was 17 when starting the first paycheck job at an architecture firm doing clerical work, drafting, and surveying. I joined SDMB at age 18 and one can see the relevance to my username.
Hopefully, I will have my first salaried job soon since I’m nearly finished w/ my B.S. degree in engineering.
AC
The odd baby-sitting job for the neighbours from about age 15 onwards. When I was about 16 I started giving maths coaching to younger students at my school. The first time I got a real pay cheque with tax deducted: when I was 17, in first year at uni, working for an insurer.
Age 15. I was a bagger/cart jockey at my local supermarket. 8-hour days on the weekend, plus one half-time shift during the week.
I made minimum wage and was forced to join a union, which took an annoyingly large chunk out of my paycheck for dues.
When I was 3, my dad and I played this WAY neato game where he would give me a bunch of envelopes and some papers, and I would fold the papers and put them in the envelopes, and make sure what I could see through the little window looked okay.
Then I got to put a cool flag sticker on the envelopes and use a rubber stamp on another corner.
It wasn’t until years later that I connected this with the monthly invoices for my dad’s trash route. 
A couple of years after that, when I was in second grade, my dad was running a storefront business, and he taught me a cool math trick to show me how to count up to $20, called “giving change”. And even COOLER than that, he let me practice it on the customers!!
My dad is awesome.
And there I was thinking I was the only kid who got to play the envelope game.
I did Work Experience (two weeks in an office arranged by the school) when I was 15. Work Experience rates were $5 a day if I remember correctly. Later that year, I did a single day of work at a fish market over Easter, washing dishes. I got paid $8/hr which I thought was amazing. That was their peak sales period and although they said they would call me in again if they ever had the need, they went broke within the year. My next job was as a sales assisant in a shoe store. I worked Saturday, 10-5:30 and got paid $50 until the government axed penalty rates, at which point my pay dropped to $33 for the same hours. I was 19 when I got my first full time job and I earnt something like $10.50/hr as a secretary.
At 14, I got paid to write some educational programs on the Apple II for elementary schools. That was pretty neat for a high school kid at the time.
At 13, I helped my mother out at her summer job on a regular basis. She worked at a seasonal gardening shop.
I got my first real job at 17 working at Fannie May. Because of it, I now hate chocolate but can identify what is what in an assorted box - a handy trick at parties featuring assorted chocolate.
Other than babysitting, I got my first REAL job at 15. I was an usher and concessionist at a mall multiplex.
I never liked babysitting that much, but my younger sister had a knack for it and my mother actually made her pay self-employment taxes (Social Security and such, she didn’t actually owe income tax as a 14-year-old). Was anyone else forced to pay taxes on their babysitting/lawn mowing money?
Fourteen. Bag boy at an A & P. Paid $0.50 per hour.
Two paper routes at 9 years old. One a free weekly, 146 papers every Tuesday, back in '83 my low monthly salary was $13, by '87 I had made a max of $37. I got fired since two little old ladies who hung about 1500 papers on mail boxes wanted part of my route, and threatened to quit. They wanted me back to do the other 80 the little old ladies wouldn’t deliver to after I was fired, bastards. The other route, a Sunday that actually payed OK, especially at X-mas, I kept for 6 years until I got my first real job in '89, 5 nights a week, 6-8 cleaning floors in a nursing home. $5.75 an hour, I was RICH!!. From the time I was 8 my father had a franchised grocery route, kind of like the bread guys, so I worked every Saturday and all summer long with my father, that is until I got my license. Guess who learned how to back up a box truck to a dock at 16? That was cool, the old man paid me well, I guess it made up for all the years I didn’t get squat. Also many babysitting, lawn mowing, construction grunt jobs, all under the table stuff through those years.
And people wonder why a 70 hour week doesn’t bother me now.