San Marino (301 AD) is often claimed to be the oldest republic in the world. (worldatlas.com/geoquiz) also lists France (486 AD),Bulgaria (632 AD), Denmark (950 AD), Portugal (1143 AD), Andorra (1278 AD), Switzerland (1291 AD) \n\nIf you define Antarctica as a country, then its borders have never ever really changed, except some minor territorial claims.\n\nMost Pacific islands have probably maintained their borders for the last 1000 years at least, if you neglect brief colonisation followed by independence e.g. Pitcairn Island, Tahiti, Federated Micronesia...\n\nI first thought Australia seemed like a reasonable guess, by default.\n\nPolitically speaking, the internal borders of England are unchanged since 1066, if you consider England separately to the United Kingdom.\n\nVery interesting discussion at second link below.