More then half subs cross their age and need to be retire
Yes, 8 Kilo and 4 Type 209 need to be replaced sometime in the not too distant future. But not all at once and the first of 6 Scorpenes will come online around 2015. Further, India issued a request for information for another six submarines in 2011. So my expectation is that all the boats will be replaced. And then there are nuclear boats too.
Meanwhile, don't underestimate the ability of submarines to 'soldier on'. While age and necessity to retire correlate in general, perhaps especially with submarines there are a good number of examples of sub classes that live on much longer than originally designed or intended, today and in the (near) future.
For example:
Spanish navy Galerna (French Agosta) class (1983-1985). To be replaced by new S-80 class per 2015-18
Spanish Navy - Wikipedia, the free encyclopedia
S-80 class submarine - Wikipedia, the free encyclopedia
RoC (Taiwan) Hai Lung (Dutch Zwaardvis Mk2) class (1987-88). No replacement expected in the near term
Dutch Submarines: The export of the Chien Lung / Hai Lung type submarines to Taiwan
RNthN Walrus class:
"The Walrus-class submarine is the only submarine class currently in operation in the Royal Netherlands Navy. They have been in use since 1990... In 2007, the cabinet approved an upgrade of the four operational vessels and recruitment of additional crew to improve overall operational availability.The upgrades are focussed on near-shore operations and integration with new weapons"
http://en.wikipedia.org/wiki/Walrus_class_submarine"
"commissioned 1990-1994... will be refitted/modernized around 2009. AIP will be one of the possible improvements. ... In March 2003 the "U.S. Office of Naval Research" wrote: "The Netherlands is working to extend the life of these ships to 2025"
Dutch Submarines: The Walrus (2) submarine class
Challenger (Sjormen) class: first commissioned 1967-68, recommissioned 1997-2001
"In 1995, the RSN acquired a Challenger class (formerly known as Sjöormen class) submarine from the Swedish Navy and another three in 1997, making them Singapore's first underwater platforms."
Republic of Singapore Navy - Wikipedia, the free encyclopedia
Sjöormen class submarine - Wikipedia, the free encyclopedia
Archer (Vastergotland) class: first launched 1987-88.
"Singapore's Ministry of Defence (MINDEF) signed an agreement with Kockums for the supply of two Archer class (formerly known as Västergötland class) submarines to the RSN on 4 November 2005. More than 20 years old and previously in reserve with the Swedish Navy, the submarines will be transferred to the RSN on completion of the modernisation and conversion for operation in tropical waters. RSS Archer was launched on 16 June 2009. The Archer class submarines are equipped with an air independent propulsion system. This enables the submarines to have longer submerged endurance and lower noise signature, enhancing the stealth capability of the submarines. The advanced sonar system allows the submarines to detect contacts at a further distance, while the torpedo system has a better target acquisition capability, which allows the submarines to engage contacts at a further range. The Archer class submarines are expected to enter service from 2010 and may replace some of the Challenger class submarines"
Republic of Singapore Navy - Wikipedia, the free encyclopedia
Västergötland class submarine - Wikipedia, the free encyclopedia
"The Södermanland Class diesel-electric submarines are in service with the Swedish Navy. They were originally launched as Västergötland Class submarines between 1987 and 1990... HMS Södermanland and HMS Östergötland... were relaunched as a new class during 2003-2004 after a major refit by Kockums...The class will remain in service until being replaced by the A26 submarines in 2018-19."
Södermanland Class Submarines - Naval Technology
Västergötland class submarine - Wikipedia, the free encyclopedia
Some new missile are still due remember 
... BTW , what year is the Indian BMD going operational ?
