Considering the franchise has been ongoing since 1978, Michael has appeared at various ages. In the original Halloween, Michael is six years old when he kills his first victim. He spends fifteen years locked up in a sanitarium, making him 21 when he breaks out and goes on the killing spree that destroys Laurie Strode's life.
In the newest films, Halloween (2018) and Halloween Kills (2021), Michael is right around 61 years old. We'll stick with this age since Halloween (2018) is a direct sequel to the first film and is now considered Michael and Laurie's canon story arc.