Due its high alcohol content, you will still be able to drink the sake without worrying about your health.
However, the recommended consumption period is usually one year after bottling. After that, the maker cannot guarantee the flavor of the sake. Whatever the flavor of your sake is right now, whether it aged well or went plain ugly, is not the flavor of the sake as advertised by the sake maker. Oh, and if the bottle is open, like wine, sake will oxidize rapidly.
Sake may or may not age well, depending on the sake. I don't have enough knowledge to say what those differences are, though from my experience, old sake can become either a tad acidic or more rich in its original flavor.
You may be able to sell the bottle so someone interested, but I don't think there is a giant market for it outside of Japan (though I may be wrong about that). In Japan, there would be plenty demand for old sake there.
By law, there should be a date on the bottle (not sure if they print that on the box) which indicates the "month of bottling". Although, that law may not have existed 50 years back...
Note there is a chance it will go bad in the case it is nama (fresh) sake, which you can tell if you see 生 somewhere on the label. Nama sake is sake that does not undergo the two pasteurization processes after being made, so the yeast is still alive. Taste guarantee is only about a half a year, and something 50 years old will probably taste yogurty or cheesy.
