The US constitution is considered to be the oldest constitution for a nation still in use and it was first drafted by James Madison.
Explanation:
The constitution of USA was to be written down, which was not the opinion of many seminal figures of the time but <u>James Madison made sure that the first draft came out of the Constituent Assembly </u>on which the Constitution was then based after a fierce number of debates and discussions.
Thus, <u>Madison's draft forms the backbone or the structure of the US constitution</u> 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