With effect from 6 April 2015, employers won’t have to pay employer’s NI on wages paid to young employees – employees who are aged under 21 at the time of the wages being paid.
It is only employer’s NI which isn’t payable, so the employee will still have employee’s NI deducted from their wages.
There is a limit on which the exemption applies. A new threshold is being created called the Upper Secondary Threshold (UST). For 2015/16, the UST is being set at £815 p/w.
More information is available from here.