Selenium cells do not age that way. Constant exposure to light does nothing permanent to them.
They only go bad when air and mositure (or whatever other nastiness you can think of) is allowed to corrode them.
So what to do is make sure that the sealed 'pod' (usually glass) they are put in isn't damaged.
What happens very often is that the leads running in and out of that sealed pod corrode and break the seal.
And then the cell doesn't survive for very long.
But other than that: indestructable.
I bought a weston master ii from a lady at the local swap meet. She gave it to me for a buck because she didn't know what it is . I have used mine for everything and it never fails me. I just set the film speed one notch lower for whatever film speed I am using and it works great. I have a nicely scanned instruction manual file if anyone wants it.
For many years, I used nothing else. I have had, and expect I can find them all if I look hard enough, a I, II, III, IV, and a V. I started with the IV in 1964, got it brand new. At one time, I started worrying about what to do, since I expected that the cells wouldn't last forever, and bought a Gossen Luna Pro F, since I was doing lots of work with flash at the time, too. I like the null reading feature a lot. I despise digital meters, digital clocks, and cameras that have liquid crystal displays with numbers on them. Gimme a dial.
Yes, the Weston meter dial is a fantastic tool. In AA's early basic photo books (the five volume set) he explores it thoroughly. At least one school I worked at had a really large version of it made of stiff cardboard for demonstration purposes.
Actually, I believe I have also a very early pre-master Weston which still has a calibration ticket. I bought it in the late '70's at a camera store in Port Townshend Washington. It appears not to have been used as the calibration ticket is attached with a string. It is a rectangular brick shaped thing (smaller than a brick, though, and a lot lighter) made of bakelite or something similar. It is laid out horizontally with the meter face on one side, and a dial on the other. As I recall, the dial isn't the same as the Master dial.
Guess I'd best have one or more of these things serviced while I still can.