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Since April 2003 the maximum hours a young person can work (a young worker is an individual between school leaving and eighteen) is eight hours per day or 40 hours per week.
 
There are rules which aggregate the hours if the young worker works for more than one employer imposing a total of 8 hours per day or 40 hours per week in total for both jobs.

There are also rules that control night work hours by young workers and impose a total ban on working during restricted hours- broadly the hours between midnight and 4am (there are slight variations to these hours under some circumstances) unless the young worker works in a hospital or similar establishment or in connection with cultural, artistic, sporting or advertising activities, when working during the restricted period is allowed.

Or if working in agriculture, retail trading, postal or newspaper deliveries; a catering business,  a hotel, public house, restaurant, bar or similar establishment or a bakery, working at night is permitted but not between midnight and 4am.
 
Remember the Working Time Regulations 1998 are to do with health and safety and particular care should therefore be taken when employing young workers.