Just to give an idea of age in relation to usefullness of subs:
In 1995, the Republic of Singapore Navy (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. The Sjöormen class were a group of submarines built for the Swedish Navy in the late 1960s. They had a teardrop shape and were capable of diving to 150m. The ships were retired by Sweden in the early 1990s. Hence, these boats were around 30 years old when acquired by the RSN. It is believed that the Challenger class were purchased to develop the required submarine operations expertise before selecting a modern class of submarines to replace them, since all the boats are over 40 years old today.
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 (despite their age).
The Swedish Södermanland class of diesel-electric submarines consist of the HMS Södermanland and HMS Östergötland. These two submarines were originally launched as Västergötland class submarines in 1987 and 1990, and have been relaunched as a new class after extensive modernization 2003 and 2004 by Kockums AB. They are 24 and 20 years old today. The newer Gotland class attack submarines of the Swedish Navy are modern diesel-electric submarines. They are the first submarines in the world to feature a Stirling engine air-independent propulsion (AIP) system, which extends their underwater endurance from a few days to weeks. This capability had previously only been available with nuclear powered submarines. They date from 1992 through 1994 and are today roughly 18 and 16 years old and remain unmodernized.
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 subs and recruitment of additional crew to improve overall operational availability. The upgrades are focussed on near-shore operations and integration with new weapons. These include the US migration from the current MK 48 mod-4 torpedo to the mod-7 version.
These are some of the more capable SSKs currently out there in service of Western and Eastern navies ...
While the French have retired their Agiosta 70s, they remain in service in the navy of Spain.
The Agosta 90B, also known as the Khalid-class, is a modernised design built for the Pakistan Navy. Various modifications give lower acoustic signature, lower diving depth, improved battery range and performance. Greater automation also allows the crew to be reduced from 54 to 36.
PNS/M Khalid (S137) - built in France by DCN Cherbourg, completed in 1999, now 11 years old.
PNS/M Saad (S138) - built in Pakistan with French assistance, completed in 2002, now jsut 8 years old.
PNS/M Hamza (S139) - built in Pakistan, commissioned 14 August 2006, now just 4 years old.
That should put the upgrade of Agosta 70B in proper perspective, I think.