pre amp tubes and gain

It is my understanding that as pre-amp tubes age, they lose gain. So one sign of tubes that need replacing is that you have to turn the volume control higher than you used to for similar listening levels.

If this understanding is correct -- which I guess is a question in itself -- then my next question is whether the reduced gain happens gradually or if it only starts to happen as the tubes near the end of their life.

So as an example: I have a tubed pre-amp (my first one, obviously) and I notice I am turning up the volume to higher levels than I used to. But the tubes are probably only halfway through their likely lifespan. So this would argue for the gradual reduction in gain theory. Or, it could just be that as I've tuned my system a bit, it is more comfortable to listen to it at higher volume levels....

Your insights are most appreciated.

Thanks.

--dan