CRB checks have been replaced by DBS checks. A caution will stay on the Police National Computer (PNC) forever but it will only show up on a DBS check if:
It has been less than 6 years since the caution was issued (2 if you were 18 at the time),
or
the offence that the caution was for is serious enough to be on the list of offences that will never be filtered.
