Oldest continuously used national flag

The oldest, continuously used national flag is that of Denmark. The current design of a white Scandinavian cross on a red back ground was adopted in 1625 and its square shape in 1748. In Denmark it is known as the ‘Dannebrog’ or ‘Danish cloth’.

Although Denmark was never part of the Roman Empire, similar designs were used by the Empire to represent provinces, as the white cross is symbolic of Christianity.
The Scandinavian cross has its horizontal stripe slightly to the left hand side of the square as you face it.