Some claim that the US has the oldest constitution in the world of any sovereign state. That is true if you define a constitution as a single document. There are much older constitutional documents like the 1215 Magna Carta (Latin for Great Charter) which remains the oldest foundation of English constitutional law. It was “the Great Charter of the Liberties” that signified the end of absolute monarchy and beginning of constitutional monarchy that persists to the present day. The UK doesn’t call it a constitution because the UK doesn’t formally recognize a single constitution. The Magna Carta is one of “the fundamental parts of constitutional law” which also includes documents such as the UK’s 1689 Bill of Rights. The UK doesn’t just doesn’t call their body of constitutional laws a “constitution”.
The sovereign city state of San Marino also has an older constitution dating back to 1600, but some Americans argue that it is separated into several books and so the US constitution is the oldest single-document constitution. This is an odd, trivial distinction. One reason their constitution is separated into several books may be that it is so old. In 1600, book binding technology was bad at binding large books into a single volume, so books were commonly divided into multiple volumes that were easier to manage. The Bible is a similar example. The bible was originally divided into 72 separate books because it was too heavy and expensive to try to combine them into into larger groupings. We still call them the books of the Bible because the Bible was originally a library of a bunch of separate books. A complete Gutenberg Bible printed on paper weighted about 30 pounds and nearly 50 pounds printed on parchment, so a single volume would have been nearly impossible to handle. As close as you can get are paper Bibles that were bound into just two 14-pound volumes and that is a huge improvement in paper and bookbinding over earlier centuries.
To say that San Marino’s constitution isn’t a true constitution because it wasn’t originally printed in one volume is like claiming that the Bible is not a true book, because it was originally printed as a library. The claim that the US constitution is the oldest true constitution is an example of the “no true Scottsman” fallacy. By any impartial definition of a constitution, San Marino has one and it is much older than the US constitution.
In any case, older is not necessarily better. Nobody views any of these documents as a model for government today. They are all a bit obsolete. The Magna Carta was written to limit the powers of the king for the benefit of other hereditary nobility, not for the ordinary people much less the slaves. The US constitution is flawed in part because it was founded on the institution of slavery. When the US has set up governments in foreign nations we haven’t recommend anything like our own form of government. That is because we know a lot more about the machinery of democracy now than we did in 1787.
When we set up a foreign government, we avoid a lot of the undemocratic features in the US constitution. We usually set up a parliamentary government which has a lot of advantages over the US system. That is what the US implemented after occupying Germany, Japan, and Iraq. We did set up a presidential republic in Afghanistan, but their constitution is not very similar to the US and Afghanistan is far from successful.