Boogie-Nicey is correct, the armed forces are exempt from a considerable amount of employment legislation , however as with other discrimination issues they will fall within the Age discrimination legislation.
That said, they will be in a very strong position to objectively justify a maximum age for many roles, due in part to the physical nature of some roles and the demanding training and return of service of others.
This is demonstrated in Australia where they've had age discrimination legislation for a number of years. For interest I've pasted the RAAF Pilot requirements below and we may see something similar, however at present no one knows what the MoD will do:
Applicants must be a minimum of 17 years of age on day of entry.
Applicants will not normally be allowed to enter the ADF until they achieve a minimum of 17 years of age, however they may be able to initiate the application process from 16 years and six months of age, depending upon the capacity of their local recruiting centre.
Based on an economic return of service, the maximum age for Direct Entry (Non-Graduate (or Graduate)) Pilot entry is 43 years of age. However, Air Force has a stated preference for pilot applicants that do not exceed 27.5 years of age at the commencement of Pilot training. Applicants older than 27.5 years of age may be considered for selection if they demonstrate significant potential for pilot training. If unsuccessful for Air Force Pilot selection, applicants may wish to consider Army or Navy Pilot entry.
Cheers