To the best of my knowledge, US police officers don't have a fixed length of service (as military personnel might have). Like most other jobs, police officers remain in service until they resign, get fired or retire. Retirement is possible from the age of 50 but many officers will continue serving beyond that time in order to enhance their pensions. See here for an example of a guy who is hoping to retire from the LAPD at the age of 50:
http://money.cnn.com/...lionaire/rp_berretta/
but here for one who is still serving at the age of 80:
http://www.thefreelib...TIVE+DUTY-a0161464791