How old is too old for a Dac?

you should go to http://www.computeraudiophile.com/ and ask there

a lot of equip. designers and serious audiophiles that specialize in DACs and streaming are there 

I see some discussion of newer DAC’s sounding better on redbook - I’ll just add that we did a comparison in my system of my Musical Fidelity A3.5 with a Schitt Modi 2 Uber and the CD player still sounded much better using its own dac than as a transport sending the digital out to the Schitt.

If it sounds good to you, than it is a good dac. 
I made the mistake of hearing an Audio Note DAC 3 Balanced and then nothing sounded good enough.

Dedicated CD transport without inner DAC will always sounds better that regular CD player when connected to the same DAC.From my personal experience regular CD players aren’t so good as transport.A dedicated CD transport doesn’t have analog circle and no inner DAC so capable to transfer a pure digital data to the DAC .

I agree that the newer DACs today are so good that there is almost no difference between red book CDs and "hi-res" .

Not to hijack, but I too am interested in upgrading my source, but honestly don't know where to start.  I have a Sony 5400es that has served me very well, but I don't know what I would have to spend on the used market on a dac to better it.  I have ARC gear, so was thinking an ARC dac would be logical, but with all this new stuff out, not sure what to do.  I'm sure at this point in time my 5400 can be bested on red book. I want balanced outputs, no more than ~2k new or used.  Can it be done? Or should I just stick with what I've got?