San Marino adopted a written constitution on October 8, 1600, and it has not been replaced since, which is the reason the country keeps turning up in lists of oldest things. The document is the Statuta Decreta ac Ordinamenta, written in Latin and arranged in six books, and what it did was put into one text the laws and customs the republic had been adding to for three centuries. It was less a new design than a written record of an old one.
The republic sits on the slopes of Mount Titano in the Apennines and says it was founded in 301 by a stonecutter called Marinus, who went up the mountain to get away from Diocletian's persecution and finished with a following. What is documented is that by the 1200s it had its own statutes and its own assembly of heads of families, and that from 1243 it was governed by two captains regent who changed every six months, an arrangement meant to keep either of them from holding the place for long. Two men, short terms.
The text of 1600 set out the council, the courts, the oath the captains took and the rules by which each checked the other. It has been amended since. A declaration of citizens' rights was added in 1974. The core is unchanged. The six-month rotation the statutes describe is the same one still in use, which means the head of state of San Marino changes twice a year.
The country got through the Napoleonic wars, through the unification of Italy around it, and through two world wars, and it joined the United Nations in 1992. Its institutions are old enough that visitors are told, correctly, that the two men at the top of the republic this month will not be there next year.