13+ to enter Y9 in UK school?

appleyummy

I always see in the admission procedures of the UK schools that applicants must be reaching 13+ yo to enter Y9.  Is it a must to reach 13 yo when entering Y9 in Sep?

E_gold8806

The children in UK enter secondary school after five years' primary education, that means one year earlier compared with children in HK. Year 9 equals to F.3 in HK, and those children in HK studying F.3 are usually 14 years old, but in UK, they are only 13 years old.