In England, you can work part time when you turn 16 as long as you either,
stay in full-time education
do an apprenticeship or training programme
take part-time classes
In Scotland, Wales, Ireland, and Northern Ireland, you can work full time at 16 once you leave school.
If you want to work at 16, learn how to prepare a CV for a 16 year old to maximise your chances of finding work quickly.