How old is too old for a Dac?

Yes, Charles: 500 dollars.

spoogemonkey it all depends on the design and your price range.  I build gear and I an tell you that you have to try different units to see what works for you.  For example standard Redbook playback with a transport versus USB or HDMI connections via your PC, or looking for DSD, then that is a different story.

OP- that is a great question.
Digital, and in particular the DAC end of things, change so fast.  I've just invested a bundle in 2 DACs and they both are supposed to be upgradable via firmware updates.  Hopefully that keeps me sane for a while

Some useful responses here. Thanks for chiming in. 

I believe that for redbook CD playback via transport or as uncompressed files from computer or server, use of an older DAC with an excellent power supply and analog output stage at a fraction of original cost seems appealing to me. Of course, with the computer use of a USB/SPDIF converter would be required.  I am about to try a used Bel Canto DAC2 in my secondary system.  Currently use a Bryston BDA-1 in my primary. 

As Bigkidz stated, considering DSD digital is different.