How do LEGO Technic pneumatics parts age?
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.