From 6 April 2015 employers with employees under 21 years old will no longer have to pay Class 1 secondary National Insurance contributions (NICs) on earnings up to the Upper Secondary Threshold (UST) for those employees.
The zero rate won’t apply to Class 1A or Class 1B NICs. Class 1 secondary NICs will apply if the employee is earning above the UST.
HM Revenue and Customs have provided some further information and examples here.
More detailed information can be found in the 2015 to 2016 help books CWG2.