Ok, since no one else has tackled this...I personally don't think I'd want to use paint that old. However, if you can get the can open and stir it well, and if the paint has a normal paint smell and consistency (separation is normal; chunks and mold are not), you could probably go ahead and use it. If you don't trust your judgement on what paint should look and smell like, take it to your paint store; they'll know. Or...shell out the money (probably $8-$18 depending on brand and quality) for a quart of new paint. Are you sure the plastic container is the same paint, not a different sheen (eggshell for walls vs semi-gloss for trim, for instance)?
Choice of sheen is partially a durability issue and partially a taste issue. I personally love semi-gloss on trim but would never want it on walls. The "trend" seems to be towards less shiny paint, especially since newer good quality paints are being formulated to hold up to scrubbing even in the flat or eggshell sheens. Eggshell or satin/pearl would be my choices for powder room walls. Good luck and enjoy your freshened up room!

Thanks jessicaml.

I decided to just stir the plastic container of semi-gloss paint because 1)it was fuller than the can and 2) my sister reminded me it wasn't 10 years old, only about 5. Looked & smelled fine and it blended beautifully.

I started the project rather impulsively but now I have a plan so I'm feeling better about it. I will not be doing the semi-gloss on the walls but will do them in eggshell like the rest of the house. I will be removing the bathroom spray that leaves all the shiny spots to prevent further spots.

I've found if the paint stirs up fine, it will usually work. If you try to stir it and get hard gunk on the bottom that does not want to stir in, or ends up really thick then I'd pass.

I bought a quart of primer from SW. They tinted and shook it up.

When I got home I opened it and went to stir, and found about an inch of hard gunk on the bottom that had not mixed in when they shook the can. It was super thick even without that hard stuff mixed in. I took it back. Salesman said it was only two years old so should have been fine. He had no idea what went wrong with it.

I did try to use some and it spread like paste, not paint. Then when it dried, it pealed off like plastic coating - ended up using some 5 year old Kiltz primer I had in the basement, it worked just fine. I bought a five gallon container of Kiltz for doing the house, and have slowly been using the last of it up.