Other than voluntary retirement, statute sets a number of mandates for retirement. Four-star officers must retire after 40 years of commissioned service unless reappointed to grade to serve longer. Four-star officers serving in the reserve active duty must retire after five years in grade or 40 years of commissioned service, whichever is later, unless reappointed to grade to serve longer. Otherwise all general and flag officers must retire the month after their 64th birthday. However, the Secretary of Defense can defer a four-star officer's retirement until the officer's 66th birthday and the President can defer it until the officer's 68th birthday. Officers that served several years in the enlisted ranks prior to receiving their commission typically don't make it to the 40 years in commission mark, because they are still subject to the age restrictions for retirement.