It all depends on what races your child will compete in and what the race organisers require.
Cross Country: Any Junior over the age of 11 on 31st August must be an Affiliated First Claim Club member of Red Rose to compete in the Mid Lancs Cross Country. Under 11s do not require affiliation to compete. The minimum age is 8 on 31st August.
Interclub: The minimum age for the Interclub races is 14 on race day. All runners must be affiliated First Claim Club members of Red Rose to compete for the club.
Other races: In our experience no under 11 needs to be affiliated but it is up to each race organiser to set their own policy.