Q: What is the oldest constitutional republic still in existence today?
A: The Most Serene Republic of San Marino.
Okay, sure — the answer to this question necessarily involves some debate over what a constitutional republic is. And you can tweak your definition to come up with a different answer. But San Marino was founded in AD 301 (which gives it claim to being the ‘oldest sovereign state’ as well), and adopted the Leges Statutae on October 8, 1600. Which is cool enough in my book to give it bragging rights to being the oldest constitutional republic still kickin’ around.
Additional fun fact: Citizens of San Marino are called Sammarinese.