The US constitution is considered to be the oldest constitution for a nation still in use and it was first drafted by James Madison.
Step-by-step explanation:
The constitution of USA was to be written down, which was not the opinion of many seminal figures of the time but James Madison made sure that the first draft came out of the Constituent Assembly on which the Constitution was then based after a fierce number of debates and discussions.
Thus, Madison's draft forms the backbone or the structure of the US constitution and thus gives the structure to most constitutions of the world. So he may be called the first person who thought of writing a constitution