While cleaning out a bathroom cupboard, I came across a stray bar of soap. From what I remember of my time here, I know this soap is many years old - over five, probably more.
Is there any reason to not use this soap?
Yes it can eventually, but modern standard bar soap should last many years if kept in a cool, dry place.
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Twelve years after moving, I found a bar of Ivory soap in an unpacked box. Opening the wrapper, it was brownish at the edges and somewhat dry, but looked OK otherwise. I used it, it seemed to work just fine.
Here’s a question, then.
What is the negative effect of using “bad” soap? Can it hurt you or does is just no longer do its job?
I have had soap smell rancid. I wouldn’t use it, but I am sure it wouldn’t hurt you. Soap is made from fat and lye and the fat can certainly turn.
Your cite is Yahoo Answers? Seriously? One of the richest collections of pure stupidity on the whole internet is your go to link? You couldn’t find anywhere else with a sufficient answer with a better reputation?
Soap that turns bad winds up in prison. Do not bend over in the prison shower to pick up the bad soap.
Speaking as someone who’s education is in chemistry, I can state definitively that “yes, it can go bad”, at least in the sense that in an atmosphere (like ours) that contains significant amounts of free oxygen, it will eventually oxidize. Depending on the storage conditions, however, it would take centuries or millenia before it stops working like soap. You are safe to use it if it’s younger than you are, or even if it’s only still in it’s original bar form.
ETA: The packaging will probably deteriorate to uselessness before the soap itself does. 
If the information offered (below) is correct and on point (and it is) I’ll link to the Onion.
Homemade soap makers would talk about DOS - dreaded orange spots. They were spots that appeared on soap made with oil that went rancid.