With a regulation I mean something akin to a constituional codex. It doesn't have to be a constitution per se, but it should outline the head of state's limitations or privileges. The oldest I have found so far is a Japanese law called Kinchu narabi ni Kuge Shohatto appr. Some regulations for the Inner Sanctum (emperor) and Aristocracy, 禁中並公家諸法度. I'm curious to know if there exists something even older out there in some other country, or if this is the first occurrence of an outside force imposing its will on a head of state.
Obviously this kind of regulation, especially if older than 1615 is no longer relevant to the constitutional status of whatever country we wold be talking about. The Japanese regulation was imposed by the warrior estate on the emperor and aristocracy, two entities that were considered "above them" (other terms used at the time are "above the clouds" and "Heaven"), hence I am interested to know if there were any regulations in other countries imposed from below that predates the Japanese version. Basically the Japanese regulation aimed to take the emperor and aristocracy out of the political process and relegate them to ceremonial roles. In Europe it was a process that, to me at least, started with the French Revolution. Thus the Japanese were almost two centuries ahead in that process. But what about other Asian or African countries, or had something similar been done among Aztecs and Incas? This I don't know and Wikipedia or Google can't really provide any answers to this question.
Why do I use the cumbersome Head of State? Simply because there were other forms than Kings or Queens at this time or before. Sultans, Khalifs, Indian Chief, Maharaja, Emperors and whatnot.