Q. Who thought of counting years from the Creation?
A. The original method of dating years was from the accession of kings of Israel and Judah. During the Persian period of Biblical history, years were calculated according to the accession of non-Jewish kings as a mark of respect for the regime and presumably in the hope that the king would treat his Jewish subjects kindly. For a short time, years were dated from the destruction of the Second Temple. By the medieval period, it had become customary to count years from the Creation.
A number of chronological works, especially the Seder Olam (attributed to the 2nd century CE), attempted to fix a date for Creation by working through the Biblical accounts of how long people lived in various generations and how many years various eras (e.g. the period of the first and second Temples) lasted.