This is a tricky question to answer! Based solely on textual evidence, the core component of the Passover seder - the haggadah - is not written out in full until the siddur of Rav Saadiah Gaon in the 10th century, and it certainly undergoes some modification since then. Other ancient versions of the haggadah can be found in the Machzor Vitry (11th century, Rhineland) and Maimonides' Mishne Torah (12th century, Egypt), amongst other places.
These haggadot are based upon the written traditions recorded in the Talmud, but are also indicative of evolving practical customs - such customs being impossible to date. The printed Talmudic material is all found in Tractate Pesachim, and comprises a commentary on that tractate in the Mishna. (The Mishnaic material achieved its final form at the beginning of the 3rd century.)
If you are interested, the relevant material is found in the tenth chapter of Pesachim in the Mishna. I have not checked the following translation, so cannot vouch for it, but here is that entire chapter as a PDF. If you would like to see this material as codified by Maimonides, it can be found in his Laws of Leavened and Unleavened Bread (Hilkhot Chametz uMatzah), chapter 8. This is a reliable translation. The following chapter, chapter 9, contains his version of the haggadah.
You might notice that I still haven't actually answered your question! Some time between the compilation of the Mishna and the formation of Rav Saadiah Gaon's siddur, we have the composition of the haggadah as we know it. It is comprised of a patchwork of different rabbinic sources - a type of midrash, in its genre. So far as the rest of the ritual is concerned, some of that is clearly pre-mishnaic (since it is recorded in the Mishna), while some of it is Talmudic. Some parts of the seder - such as the singing of certain songs - are later still. (The Aramaic song, Chad Gadya, is actually based on a German folk song, for example, and is clearly written at such a time as Aramaic was not spoken.)
If you would like to read more, I would recommend the introduction to Yosef Hayim Yerushalmi's Haggadah and History.