The requested information is not available.
If a child has been part of a claim, but is not part of a claim when they are 15 years, 9 months old, HMRC can be contacted to request a NINo.
If a child has never been part of a Child Benefit claim (or a claim for a childcare service administered by HMRC) HMRC will have no record of them. An application can be made to DWP to obtain a NINo.
HMRC do not collate information on the number of young people who fall out of the automatic process and do not later obtain their NINo.