It depends what you count as a first job!
From the age of 15 I worked myself, looking after people's cats / dogs / ducks / ferrets etc. when they went on holiday. Pay wasn't done ph, it was per job (and that varied a fair amount!) but it worked out pretty well - often £20+ph, although I didn't work full hours.
At 18, I had my first "proper" job - it was with a hospitality agency. I was on adult minimum wage (£5.92 at the time I think). I did two shifts with them (one waitressing, one bar), before they went bust!
And finally my first full time job, behind a bar, at the age of 18.