Cant get my rear derailleur to reach all cogs I have been having an issue on my road bike getting the rear derailleur to reach all of my cogs.  I can either not reach my two smallest cogs or two largest cogs depending on the cable tension.

I found a nice set of instructions here: http://www.williambenedict.com/bikes/derailleurs.html and followed through them with the cable disconnected and I was able to set the limit screws so that if I moved the derailleur manually I was able to shift smoothly to every cog.  Once I connected the cable I was able to shift to all but the 2 largest cogs so I started adjusting the barrel 1/4 turn at a time.  I had to turn the barrel almost all of the way in order to get to the 2nd largest cog(I can't even reach the largest) and with this adjustment I can no longer reach the two smallest cogs.  

From what I can tell my derailleur isn't bent at all.  Also, I'm not sure if it is much more difficult to shift to a larger cog than it is going back to a lower (I have bar end shifters).  I have to pull on it quite hard.

Since I am able to successfully shift to every gear smoothly whilst manually moving the derailleur, I imagine this is an issue with my cable.  Would it's age have anything to do with it?
As you suggested, the cable may be at fault.  I change my gear cables once or twice a year to keep shifting smooth and free. If the cable/cable housing are old and worn they could be offering resistance.  If that resistance is too high the springs in the derailleur may not be strong enough to pull the cable through.  

Try removing the cable and housing, then make sure the derailleur and shifter are moving freely before fitting the new cable.

Then replace the cable and housing (these are cheap to buy online, but you do need a pair of housing cutters to cut the cable housing - think mine cost be about £15).