Myself and others from the math schools knew this algorithm in 8th grade for sure, as we been already using it in 9th grade for competitions. This does not mean all my classmates knew it, of course not.
So it depends who you teach this to. Theoretically - you should be able to, practically - well, perhaps not so much, as math is not the only thing 8th grader learns, in fact his head is bombarded with ... dozen disciplines at a time.
Besides, I recently met a classmate, previous IoI medalist, who works quant-something somewhere for 15th + years, and is a PHD and everything. We start talking about mathematics and I find to my total surprise he knows very little about grammars, never used them. He remembers Dijkstra or Ford-Fulkerson, but only as a title, while I'm sure he learned these at some point in Stanford, as the shortest-path and A* was not something we had in textbooks back in the 90s for sure.
Merge sort is supposedly invented in 1950, it’s more likely it was invented in 1050 than 1950. Sort a room full of documents for me. You have three minions, go.
given Al Kwarizimi lived ~12 century (if memory serves right) it is a fairly old term in this regard.
when was it that modern people started using the word, I'd bet beginning of 20th century, but this is a wild guess.
when did everyone started calling algorithms "ai", well perhaps ~1960 ?
But a room of five clerks all taking tasks off a pile and then sorting their own piles is merge sort at the end of the day. Literally, and figuratively.