How do LEGO Technic pneumatics parts age? I recently bought LEGO Technic 8868 (Air Tech Claw Rig) and 8460 (Pneumatic Crane Truck) models. I noticed that the pneumatics tubes seem to have been changed since the colors are different from the instructions manual.

Since these models are respectively 24 and 21 years old, a possible explanation is that the original tubes have been lost.

But it may also be due to aged tubes that needed to be changed. How do tubes change when they're ageing? Do they become less flexible and break when they're failing? What is the lifespan of these tubes?

It looks like the pneumatic cylinder, pumps and valves have not been changed and they still work. So I think that they can last longer than tubes.
The tubes generally age very well and I have never experienced a problem with them (I have had several of these sets over the years). 

I would double check that the pump is working correctly and you have definitely not got a poor connection somewhere (a leak). 

Double check also for blockages in the hard plastic dark old grey connecting tubes that are clipped on and the 'T's. These have a smaller inner diameter than the rubber tube and would be the first to get a blockage if something has got in to the system. 

If you are certain its a tube problem you can buy replacement tube online. I would carefully double check for another problem before buying tube as the tube rarely fails unless it has been exposed to chemicals or a lot of sunlight.