IOS

iOS 6.1 (2013-2015)
iOS 6.1 is still supported and its still the latest version today for iPhone 3GS and iPod Touch (fourth generation).

iOS 7.1 (2014-present)
iOS 7.1 is still supported and its still the latest version today.

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iPhone OS 3.x (2009-2011)

iOS 4 (2010-2012)

iOS 5 (2011-2013)

iOS 6 (2012-2014)

iOS 7 (2013-present)

iOS 8 (2014-present)