According to the AFCO 5 Guidance notes

Age on application - 17 years.

Age on entry to service - Dependent on service and specialisation (I take that to be for the likes of UKSF(R) and some of the more specialised jobs such as units requiring HGV drivers etc.
Why not try the Royal Marines Reserve - apparently they've recently started taking people from 16.

Off the website:

"Applicants can start the joining procedure from the age of 15 years 9 months; but must be at least 16 years of age on entry and have completed their mandatory secondary education."

i.e. finished GCSEs.

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