Old Korean would describe the language spoken in Korea prior to the 15th century. Little is known about it. This period of Korean history is known as the Three Kingdoms period, followed by the Unified Silla period – Silla being one of the Three Kingdoms that dominated the Korean peninsula, and the kingdom that wound up surviving and thriving. China had a huge influence on Korea during this time (as it continues to do today) and our few examples of Old Korean are actually written in Chinese characters. This makes deciphering them difficult, as the Chinese characters appear to have been used to represent the sounds and meanings of the Korean language at the time, as opposed to an actual translation. This makes understanding them very difficult.
The evolution from Old Korean to Middle Korean is also argued – but generally it’s agreed that by the 15th century, with the advent of a Korean-specific writing system, the Korean Language had unified into what is known as Middle Korean. This writing system was called The Hunminjeongeum (which translates beautifully to “The Proper Sounds for the Instruction of the People”), known today as Hangul, was made official by royal proclamation on the part of King Sejong the Great. This script included pronunciation rules and was intended to allow the largely illiterate peasant population in his kingdom to be able to read and write. It was believed to be promulgated on October 9, 1446, and that day remains Hangul Day in Korea.
From there Korean follows a more standard and better-documented evolution model of language. Unification and standardisation continued until Modern Korean was established as distinct from Middle Korean by the 17th century.
Since the division of Korea into North and South, the language as spoken by each country has begun to drift apart, although they are still firmly one language and mutually intelligible. What’s really interesting about this is that much of the changes in North Korean are do to government intervention, as North Korea’s communist leaders have issued several proclamations concerning the language – a rare situation in language evolution, which is usually much more of a natural process.