NIM39320 | National Insurance Numbers (NINOs): Registration of a new NINO: Juvenile Registration: exclusions from the automatic process
From HM Revenue & Customs · National Insurance Manual
Some young people may not be included in the automatic registration process. See NIM39305.
Providing they are over age 15 years and 9 months they should
use the "find my NINo" digital service to trace their National Insurance Number or
contact the National Insurance registration Helpline on 03002003500.
Children who have not been part of a Child Benefit claim or a Tax Free Childcare award will need to apply through DWP. For details see NIM39325.