Michael Myers is a fictional character in the Halloween film franchise, created by John Carpenter and Debra Hill. He first appeared in the 1978 film Halloween and has since become one of the most iconic and recognizable horror movie villains of all time. Despite being a fictional character, the age of Michael Myers is a topic that has been discussed and debated by fans of the franchise.

In the original Halloween film, Michael Myers is portrayed as a young boy who murders his older sister on Halloween night in 1963. At the time of the murder, Michael Myers is just six years old. He is subsequently institutionalized and spends the next fifteen years in a mental institution under the care of Dr. Sam Loomis.
In the 1978 film, which takes place fifteen years after the murder of his sister, Michael Myers is portrayed as a fully grown adult. His age at this point is therefore twenty-one years old. This age is consistent with the age of actor Nick Castle, who played the role of Michael Myers in the original film.
In the sequels to the original Halloween film, the age of Michael Myers is somewhat more ambiguous. The events of the sequels take place over a period of several years, and the timeline is not always consistent. In some of the later films, Michael Myers is portrayed as being middle-aged or even elderly, despite only being in his early twenties in the original film.
One possible explanation for the discrepancy in Michael Myers’ age in the Halloween sequels is that the character may have aged at an accelerated rate due to his traumatic experiences. It is also possible that the filmmakers simply did not pay close attention to continuity when creating the sequels.
In the 2018 film Halloween, which serves as a direct sequel to the original 1978 film, the age of Michael Myers is once again consistent with his age in the first film. The film takes place forty years after the events of the original Halloween, and Michael Myers is once again portrayed as being in his early twenties.
In conclusion, the age of Michael Myers from Halloween is six years old when he murders his older sister in the original 1978 film, and twenty-one years old when he is first portrayed as an adult in that same film. While the age of the character is somewhat more ambiguous in the sequels to the original film, it is generally consistent with his age in the first film. It is also worth noting that the age of the character may be somewhat fluid due to the traumatic experiences he has endured, and the filmmakers may not always pay close attention to continuity when creating sequels.