How old is too old ?....

16yrs is really way too old. Go buy new ones immediately. There, that's what you wanted to hear.
If I were you I'd listen to some new B&W and Kef reference (amongst others) and decide whether it's worth it.

I have this exact same dilemma. I have upgraded my electronics, and then wondered if the KEF 104.2s were worth upgrading. Luckily my friend had a pair of new B&W CMD7 NT. These are the same price as the 104's were back in the early '90s, so with inflation the B&W are actually cheaper today. The reason for using these speakers is that I thought they were in the "same league" as the 104.2 in their day. Anyway, we brought the speakers over, spent about an hour dialing in their location in my room with my electronics, and the results surprised me. The stage, the depth, the accuracy, and overall reality of the presentation was far better in the B&W’s. As a long time fan of these speakers, I didn’t want to believe it, but it was true. The technology has gotten better, no doubt. The high quality electronics that you own, you are not even hearing yet, trust me. Take you 104’s and may use them as a start to a surround sound system, and get a new pair of speakers, so you can finally hear your “system”.

I have the KEF 107s and just, not too long ago upgraded my front end (CD Player and Pre Amp.) After upgrading the front end I went out a picked up the Dynaudio 1.3SE and the difference is night and day, don’t get me wrong the KEF 107 do have the deeper bass, but that where it ends. To me the Dyns are just a better speaker . the highs are sweeter, the soundstage is wider and deeper, the bass also is still very good for a monitor speaker. Up grade your speakers you will feel go about it.

I have a pair of 104/2's and had them rewired and had the caps replaced in the crossovers. The foam baffles were a tough thing for my repair shop but they solved this too.

They now sound wonderful. I know that they are a bit "looser" than modern sets, but I am very pleased.

I have had two pairs of Kef 104/2's. I agree with alot above in regards to the"parts" of good sound. The bass may be better in newer speakers(you can add a Rel Sub) Te trebel may be sweeter and the soundstaging and imaging better.
But one thing the newer speakers can't do- and I've tried the following NHT 2.9, Audes Blues, Revel F30's, VonSweikhart Vr-4 JR Triangle CLeius, B&w 802/3,4. The one thing they can't beat is the soul of music which the Kefs do beautifully. And I think you know that.