The military's new blended retirement system goes into effect Jan. 1, 2018. All currently serving service members are grandfathered into today's military retirement system. Service members with fewer than 12 years of service as of December 31, 2017 (or fewer than 4,320 retirement points in the case of reserve component members), will be able to choose to remain in the current system or opt into the blended military retirement system. Also, if you joined the armed forces after December 31, 2005, but before January 1, 2018, you will have the choice to enroll in the Blended Retirement System or remain in today's current retirement system. If you joined the military before January 1, 2006, you will be grandfathered and remain in today's current retirement system.
For more on military retirement pay, see this Military.com Military Retirement System Overview.