Does tube testers tell how much life is left for tubes?

I use the uTracer and i am happy with it. Most important for me is transconductance.Only in German, sorryhttps://www.orangeaudio.de/blog/2021/04/29/roehrenmessung-mit-dem-utracer/

An approximation of tube life can be found from the cathode emissions, which I think some testers can provide.

When the cathode electron emission drops way low the tube is at the end of its life.

Tubes can test good right up to the end time of failure.

RIght, the testers can tell you whether or not the tube is OK. They can not tell you how much time is left on the tube until the tube starts to measure poorly which means the tube is at deaths door. I use tubes until they fail and in my experience they sound just fine until they do fail. 

I always keep a full set in reserve. If one tube fails I replace the whole lot unless that particular tube was obviously defective. I keep my phono stage on in standby. The tubes, 6922's last an average of 6 years.

different tubes age in different ways

6dj8 6922 types, in my experience, get noisy with tube ’rush’ or hiss, and microphonic long before they quit entirely - not sure why this happens in the physics occurring inside the tube as time passes -- these will test fine on the tester for gm but then plug em into the linestage or phono stage and they are unlistenable

depending on the application, that change defines their effective ’use-ability’

other tubes may age differently... typical power tubes usually get dull in sound (lose treble detail imaging etc etc) as they get into last 30-40% of their lives