GLX_EXT_buffer_age is an important GLX extension particularly for compositing window managers. NVIDIA's binary Linux driver has supported the buffer-age extension since late 2012 and it's also been found in AMD's Catalyst driver since 2013. There's also been Mesa code for the buffer-age extension since last year for DRI2 but now it's been brought into the DRI3 world.
As explained by the OpenGL.org registry, "The aim of this extension is to expose enough information to applications about how the driver manages the set of front and back buffers associated with a given surface to allow applications to re-use the contents of old frames and minimize how much must be redrawn for the next frame."
The DRI3 buffer-age work was done by Adel Gadllah and merged this afternoon.