General Sergei Karakayev, the Commander of the Strategic Rocket Forces, announced that the R-36M2 missiles (SS-18, RS-20V) will stay in service until 2026. According to Karakayev, service life of these missiles is extended to 33 years.
Normally, missile service life is extended incrementally based on results of flight tests. The last time the Rocket Forces tested an R-36M2 missile, in December 2009, its service life was extended from 20 years to 23 years. This time, however, it was increased to 33 years without the benefit of a flight test. The long life of a missile is not unusual, though - UR-100NUTTH/SS-19 had its service life extended to 33 years in 2008.
UPDATE: Karakayev actually said that the service life of R-36M2 has not been extended yet. It could be extended to 33 years as a result of a program that would include a series of measures developed for this purpose (which apparently would include flight tests).