I like how wikipedia goes about this question: Must be one of the 193 current sovereign state Go by the date that the nation itself accepts as its symbolic starting point
China is indisputably one of the oldest. It was unified by the Qin Dynasty in 212 BC and has a line of dynasties that can be clearly traced to today. (The Communists don't call themselves a dynasty but that's what they are).
Wikipedia puts Iran as being older than China, saying that the proto-Elamite kingdom on the Iranian Plateau in 3,200 BC was the beginning of that nation. Seems like a stretch to me, but I don't know enough about Persian history to make a judgement.
Image: https://www.quora.com/
