Well, I think we must first decide what religion means. There are two definitions which can work here. I will quote from a dictionary so I don't distort the definitions with my own words. My source is: Thefreedictionary.com. 1) belief in and reverence for a supernatural power or powers regarded as creator and governor of the universe. 2) a set of beliefs, values, and practices based on the teachings of a spiritual leader. Next, we must decide where man originated. I believe Adam was the first man. And since the flood, we can't be certain of the exact location of the Garden of Eden. For the sake of time and space, to pursue this subject further visit answersingenesis.org and search for the article "Was the Garden of Eden Located in Iraq?". Very enlightening reading. Arbitrarily accepting it's location to be somewhere to the Middle East, I'll move on. If we use the first definition of religion, then the logical conclusion would be the first religion was the oral law given to Adam and Eve from God. To use the second definition, the spiritual leader being a man, by implication, would have to be written. Therefore the first religion would be the laws given to Moses by God - the Ten Commandments. Whether these things can be proven or disproven by empirical evidence such as archaeology, I suppose is up for grabs. Would a more pertinent question for us today be "what will be the last religion?"
Try to understand or discover what does religion really define to you. Since there is only one kind of religion. Most people do not know this. There is one example that God accepted the kind of religion which found in James 1:27. Try study it and see how much different is it then what the worldly men defined religion.
Nobody knows. There was no record of it.
Hinduism is believed to be the oldest existing religion.
Probably some form of animism
Despite Jewish claims which predate their actual history by several thousand years, it's generally acknowledged that the first recognised religion is Hinduism, which is at least 5,000 years old.