If the potato is just old and doesn't smell bad or have any other bad spots on the potato then the potato is safe to eat.
If the potato smells bad or is really mushy and turning moldy then the potato is bad and should be thrown out.
If the potato is just old and sprouting then it's safe to eat.
The sprouting of the potato doesn't mean it's bad although it may not be as good tasting.
If the potato is really bad enough and you eat it the bad rotten potato could cause solanine poisoning.
The solanine poisoning can make you very sick and cause symptoms such as vomiting, diarrhea, headaches, paralysis of the central nervous system.
In some cases the solanine poisoning can cause death.