A time delay on switch on is an excellent feature IMO. The biggest threat to long life is thermal shock and voltage surges.
The hotter a component gets, the shorter it's life. The issue which we all live with using tubes, is they heat up and cool down, and basically burn themselves out. The life of a tube is allied to the construction quality, age does not seem so critical IMO, hours of use and how hard the tubes are pushed in the design (volts on the cathode).
Some manufacturers recommend leaving tube amps on all the time (pre-amps) like Audio Note. Power amp tubes that is not a good idea or even safe to do that. My experience with tube amps, as long as you watch at start up for flares or shorts, and listen for any noise, treble harshness or flabby bass indicating a failing tube, you should be ok.
Some of these 'new manufacture' classic tube copies may (it is reported) have premature tube structure collapse. That obviously is a serious issue, as it could take part of the amp out by causing a short. That would require sending back to the manufacturer, as I had to do with one of my 300B Monoblocks. It cost nearly £1000 for the repair as out of 2 year warranty.
My love / hate relationship of tubes, it nearly kills me! I love the sound, hate the hassles. Right now, I still have tubes in my DAC, but have dropped the tubed power amps and headphone amps for SS instead. I feel I have not lost much, maybe 5% at most in overall SQ, some parts not quite as good, other parts (dynamics and bass) better. It's like that this hobby, fascinating and fun. Her in doors hates the tube gear BTW which doesn't help the cause...... boxes of tubes in the post, amp repairs, hassles etc.
