Does phosphor(especially halophosphate) notably age even if the lamp is not used? When I last time visited my familys summerhouse I noticed that lamp of small 15W fluorescent fixture in a storage shed looked really worn out. At that point I just thought that I did not have any spare lamps there as its the only 15W fixture there. Later when I was in sauna, that thing came back to my mind and I started to wonder why that lamp looked so worn out as it could only have some hundred hours of use. I bought that lamp about 10 years ago, at the same time I installed that fixture too. and because our summerhouse is not connected to powergrid(only generator) it could not have that much of use. Phosphor of the lamp looked like dirty with darker spot in it and dimmer than it should. the lamp is Airam F15T8 kirkas valkea (CW halophosphate lamp). well, I guess I have to replace the old lamp.

You wrote, the house is not connected to power grid. How is the fixture supplied? As many low DC input inverter ballasts tend to severely underpower lamps without providing cathode heating. The result is mostly cold-cathode operation, what yield to very quick sputter.

Other thing: Did you tried to run the lamp for longer time without interruption? Because when long time not in use, internals settle in the way they are not supposed to during normal operation, as typical recovery mechanisms are not in place (e.g. mercury distribution). Particularly the mercury "fog" on the inner surface lead to grayish tint and dim light after long OFF time. But the lamp can usually fully recover (after it evaporate), the lamp simply need to be burned some time (at full power) for the "cleaning" to take place.

And last and not least, lamps tend to be simply dirty, even when used in enclosed fixtures (no fixture is tight enough to avoid this). A cloth with soapy water help - but you should avoid wetting anything else then the pure glass tube and let it fully dry before installation into closed fixture.

there is a solar panel system and a gasoline powered generator for 230V. this particular fixture is a 230V fixture with magnetic preheat ballast. The dirt might well be just a lack of use as you said. As its in a storage shed its only used sort amount of time at once. I will try to run it a longer time once Im next time there and see does it have any effect(+ physical cleaning is always good to do once in a while). So the phosphor itself does not age even when the lamp is exposed to winter time enviroment and other "not steady storage enviroment"

i doubt the phosphor degrades.its in a vacuum with argon/neon/krypton gas and a bit of mercury.
i did think the original tube from a my grandparents 1938 desk lamp had dead phosphor till it ran a while.just condensed mercury.i got it in 2002.
not as bright as modern tubes but it still works.

It could also be mercury in the phosphor, I found a couple of ancient tubes in an old DIY shop hidden away. They must have been about 15 year old and when I first hooked them up to the ballast they flickered dimly and took about 15 minutes to reach full brightness. While they were warming up the grey colour of the phosphor started disappearing at the cathodes and spreading along to the centre of the tube until it reached full brightness.

Since this first switch on they've been fine!

i doubt the phosphor degrades.its in a vacuum with argon/neon/krypton gas and a bit of mercury.
i did think the original tube from a my grandparents 1938 desk lamp had dead phosphor till it ran a while.just condensed mercury.i got it in 2002.
not as bright as modern tubes but it still works.

Phosphor DOES degrade when a lamp is used.

i know that.we were discussing degradation in long storage.i suspect the 1938 tube has a lot of wear as it has heavy end blackening.

It could also be mercury in the phosphor, I found a couple of ancient tubes in an old DIY shop hidden away. They must have been about 15 year old and when I first hooked them up to the ballast they flickered dimly and took about 15 minutes to reach full brightness. While they were warming up the grey colour of the phosphor started disappearing at the cathodes and spreading along to the centre of the tube until it reached full brightness.

Since this first switch on they've been fine!

I have had lamps to do this too.  I got many of my Miller fixtures from the old cotton mill.  Some of them had not been used for many years and had nearly NOS Sylvania Lifeline F40/D lamps in them.  When first powered up, they were shimmering and dim in the middle.   Once run a few minutes, they came up to full brightness and have worked fine ever since.  This seems even worse with the newer "low mercury" lamps.