With more than 25 years, you can use it in good condition. What else tube amplifier could be that way? I do not know.
Most competent amplifiers will go 25 years with ease. The ones that can't make 25 years are likely to put the producer of such amps out of business long before 25 years has gone by!
@ihcho If you want to get the best out of a tube amplifier its really in your best interest to use a speaker that is 8 Ohms or more. Speakers that are hard to drive really don't show off any amplifier (tube or solid state) all that well.
This is simply because the harder the amp has to work, the more distortion its going to make. On top of that, with most tube amps you can lose up to an octave of bandwidth off of the bottom end by using the 4 Ohm tap as opposed to the 8 Ohm tap; and you need that 4 Ohm tap to work with 4 Ohm speakers!
You may recall that I advised you earlier about running a speaker of lower sensitivity; I said that 100 Watts would likely not be enough. What I didn't say is that you really want any amplifier to be loafing while doing its job.
Speakers don't have to be inefficient to have excellent definition! My speakers are 98dB and are some of the more revealing speakers I've heard. You also get greater dynamic contrast as the more efficient the speaker is, the less thermal compression it tends to have.