From the Avon and Somekit police web site
You may apply for the position of police constable on reaching 18. However, you must be 18 years and 6 months old before you can be appointed.
There is no upper age limit (this is subject to change) but please bear in mind that the normal retirement age for police constables and sergeants is 55 years and that new recruits are required to undertake a two year probationary period.