I've got some from ~25 years ago that are discolored due to exposure to sunlight. There are ways to fix that (http://lego.jtooker.com/whitening/). I haven't noticed them getting brittle, though.

I've also got some bricks that are much older (from the 60s I think), and are still usable. They haven't spent time in the sun, so they are not even discolored.