By definition a terrorist is someone who uses violence to achieve a political goal. We generally don't use it to refer to state actors involved in a war, but the Korean War was not a war - it was a police action.

The US/UN did not recognize North Korea as a state - they had boycotted the UN sponsored elections in 1948 so they were not recognized as a state actor. (To this day, North Korea is still not recognized by a handful of countries. South Korea is recognized by all EXCEPT North Korea.) Their attack on the south was repelled with a police action. A police action is a war except without the declaration. In the US, politically, this gives the President and the military a bit more freedom from Congress. It wasn't until the War Powers Resolution of 1973 that Congress stepped in to check the President's powers in this regard.

So yes, North Korean fighters could legitimately be called terrorists. They had aggressively attacked someone for political change.

As to your question about when, take a look at this Google n-gram. I included the words terrorist, terrorism, anarchist, and anarchy. You can see that terrorism had a big bump in the 70's and then a huge surge around 9/11. In the 70's, there were lots of plane hijackings. Here is another n-gram of the words hijack and hijackers and you can see a big spike in the late 60's through the 80's, and another surge in the early 2000's. There was also the rise in Middle East violence in the 70's and 80's - the Yom Kippur War, the Beirut bombing of the Marine barracks, Libya, Pan-Am 109 - as well as The Troubles in Northern Ireland. All of these things involved non-state or non-recognized state actors. Again, after 9/11, that huge surge in the use of those words is tied to non-state actors. I included the words anarchy and anarchist because those were big movements in the late 19th and early 20th centuries (McKinley's assassination, anti-Japanese fighters in Korea and China sometimes were tied to anarchy movements, and the semi-close ties between anarchy and communism) and you can see that as those words died out, terrorist and terrorism started to rise.

So to answer your question, terrorism as a word has been around for quite some time. Like all words, the meaning has shifted with people today more likely to think about Middle-Eastern terrorism instead of Leon Czolgosz, but terrorism is simply the use of violence to achieve a political goal. North Koreans (and the Chinese involved too since they were officially volunteers and emphatically not a part of the official Chinese military /s) could definitely be called terrorists. Had the Korean War been an official war between state actors, I'd wager you'd be a lot less likely to hear the term used (how often do we hear people call the Axis powers "terrorists"?), but by the definition itself, it would still be possible.

TLDR - terrorists are people who use violence to achieve a political goal. The word has been around for quite some time but generally is used to refer to non-state actors.