Corby, Class 1 NIC's apply only to employees who are aged 16 or over. Employee contributions stop at State Pension age, but employer contributions continue for as long as the employee continues to work.
I was also using the HMRC Employers Online SSP calculator, which would only make a calculation if the box indicating that employer's Class ! NIC's were paid, otherwise a form SSP1 would have to be issued. The employer would not pay any NIC's should the wage be below the LEL.