How do you know if its old:
Let's take a look at these two unsifted matcha, a and b.
Ignore the difference in color, because variations in lighting can largely account for the difference. Look at the texture. Matcha a looks coarse, hard, lots and lots of clumps, where as matcha b has a lighter, fluffier look.
I'm still new to matcha, but it seems to me that there is a big difference. Mathca a did not have a vibrant or fesh taste that I usually get with matcha. In my opinion, tea is "fresh" as long as it tastes good.
Should you refrigerate. I don't think so. I might refregirate an unopened can, and if I did, I would leave it out of the fridge for several hours till it reached room temperature before opening it.
Matcha often comes in either 20, 30, or 40 grams. I've seen 100, but unless you use a lot, I wouldn't buy that much at one time. A serving of usucha, thin tea, is about 2 grams, so you only get 10-20 bowls of matcha from one container, less if you're making koicha, thick tea. You should be able to finish a tin of matcha long before it gets too old, so in my opinion, the risks of storing an opened container of matcha in the fridge far out-weigh any potential benefits.