Depends on the job ^^ I'd say 16 though.
As Zayne says, it depends on the job. My father bought a new lawnmower after I finished the 6th grade. He paid for the first gallon of gas each year, a can of oil each year, and had it serviced including blade sharpened each year. That was to pay for the mowing of our lawn. If I wanted spending money, I had to mow other people's yards. Lawn + trimming around house, sidewalk, and driveway was $1.00 for most places, I did have one large yard I did for $2.00 (1969-1972).
The first job can start from being 12-13.
I started babysitting at aged twelve. By the time I was thirteen, I was babysitting for four families on a regular basis - two of the families had four kids each.
When I turned sixteen, I got a "real" job working at a movie theatre.
I would encourage everyone who is physically able to, to begin working as soon as they can manage family, school and work. Nothing prepares you more for life than having a job that has some teeth to it - one that teaches you how to behave in public, how to count change the correct way, one that teaches you that life is not fair and one that gives you money for the work that you do.
That's when I got my first job.
Like Bruno said, I had a paper route at 12. I donl;t think those exist for kids anymore.
My first "real" job with a paycheck and deduction and all that was 16, unloading and sorting plums in a packing shed.