Do stocked tubes go rubbish over time? I'm stocking tubes that I found in discount. One of my friends said that tubes get old over time. Do Butyl tubes do this?  If it is true, what are storage tips for stocking them? 
And they came packed with a powder-like solution, how does this work?
Yes, they do degrade over time. Unfortunately, various brands and different storage conditions yield different results, so I'm not able to give an estimation of how long can you safely store a tube. How ever, I can recommend the conditions that appeared to give best results.

The tubes I could use after long time of storage without problems where those that I kept in my room, which being in a tropical country, had temperatures of 15°-25° Celsius. They where obviously far from paints, solvents or oils, where taken out of bags and kept on a small plastic basket. Those that came in a cardboard box, where kept in the box. Also, at buy time, they where dark gray and with a subtle shine.

Negative results where obtained with tubes that where kept outside the house, in a dusty environment and subjected to humidity but not direct sunlight. (And there were paints, oils and solvents nearby, though not in an enclosed storage)

Also, tubes that had dull surface and where very dark (i.e. black) at buy time where the ones that failed shortly after installed, almost regardless of whether they where used immediately or after a time stored. Due to this, I suspect that they where already chemically damaged (And today I'm reluctant to use/buy a tube that looks this way).

Another observation is that Tubes I stored inside of plastic bags where found stuck to themselves and with a sticky or gooey surface after a year or so.

Upon inflation (outside a tire) those tubes that where badly stored shown one or more of the following tale signs:
 - They stretch irregularly, with bulges, instead of uniform diameter.
 - Crackled surface.
 - Irregular surface (different color and/or texture in certain spots).
 - Apparent punctures, i.e. what visually appeared to be holes but did not leak when water tested.