Very old cans of paint will probably rust away before they dry up,
esp. latex.

Related story:
I painted a client's front door with an old can of latex paint she
provided. I gave it a good shake and stir, then started brushing. After
a few minutes, I mentally kicked myself for dripping paint down the
side of the can onto her concrete porch.

I picked up the can to find a big puddle underneath. The bottom of the
can had rusted through in a pinhole, and the paint had cured around the
leak, forming a scab. My mixing opened up the hole.

I asked the client to leave the puddle in place. Two weeks later, I
peeled it off like a pancake. The only issue was that that spot on her
porch was cleaner than the rest. :-)

Lesson learned: Set paint cans on the plastic lid of my tool caddy.

--
Steve Bell
New Life Home Improvement
Arlington, TX USA