So I wanted to check who's had their current flag the longest. However I quickly learned that that's not such a straightforward question, seeing as there's (1) the first recorded use of the current flag design, (2) the official date of adoption of the current flag design, and (3) the most recent change to the current design, whether small or large. In the end, for the creation of this map, I simply went with whichever of those 3 dates were the latest. My idea being that America's flag in 1777 wasn't characteristically the same as the 2015 flag, even though the "design" is the same but has just been modified most recently in 1960 via an added star.
If you really are curious about all the different dates, here's my wikipedia source.