They may still be competent magistrates at age 70 (although not such competent poets) but there has to be a “cut off” age to give younger people opportunities.

Age 70 is higher than the average company retirement age (65) and if judges have a retirement age of 75, that reflects their specialist position and limited number of suitable applicants.

Where these magistrates absolutely have a point, however, is in criticising the justice system for ageism. Retirement is mandatory at 70 for magistrates. Is the Ministry of Justice, which sets the limit, saying JPs can’t make proper judgements when they get to this age? And if so, why are Crown Court judges and members of juries who have to handle more serious cases, such as murder allowed to go on until 75, when magistrates aren’t deemed fit to pass sentence on shoplifters?

http://www.independent.co.uk/voices/comment/if-you-can-be-a-ruler-at-70-why-not-a-local-magistrate-10245224.html