How old is too old for a Dac?

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. 

H I Jond,
What differences are you hearing via further listening to the Audio Note and the Yamamoto DACs?  I'm assuming this was a fun and informative comparison. 
Charles 

Charles to be honest the AN keeps getting better by the day I haven't been able to take it out yet! I I am pretty convinced the AN is considerably better than the Yamamoto but hearing being fickle I will give the Yammie one more listen before I decide.  Off to NYC for the weekend so that will likely be next week or weekend.  There is just something so right about the AN though I feel very fortunate to have won that auction now!
Cheers,
Jon