This is how HMRC explain the change: “the rate of employer Class 1 secondary NICs for employees under the age of 21 will be 0% up to the new ‘Upper Secondary Threshold’ (UST) which, for the tax year starting 6 April 2015, will be the same as the Upper Earnings Limit (UEL). Class 1 secondary NICs will however continue to be payable on all earnings above this threshold. The basic rules and calculations of National Insurance including how Class 1 NICs are assessed will not be changed by this measure. Class 1A and Class 1B NICs are not affected by this change and should be collected and reported as normal. For employees who are at, or over the age of 16 and under the age of 21 there will be a range of new NI category letters to use when assessing secondary NICs in respect of the earnings paid to these employees. From 6 April 2015, when submitting PAYE information for employees under the age of 21 you will need to use the new category letter appropriate to the individual.”