According to our friends over at
Wikipedia...
QUOTE
The Dannebrog, national flag of Denmark, is the oldest state flag still in use.
The Saltire, national flag of Scotland, is the oldest national flag still in use.
Others say...
QUOTE
The Red Dragon flag of Wales is the oldest flag in the world (the second oldest is Denmark's). Some say the Romans brought the Red Dragon there, some say that it grew out of the Druids' veneration of the serpent or dragon (Wales was a Druid stronghold) and some say that it is an echo of the dragon legends to be found in the mythology of many lands across the world, surviving in just two of them - Wales and China
Other 'Oldest' flag information
here.