Throughout "Halloween", different years are given that, if added correctly, would make Michael Myers 21 years old (his true age). Also, someone says that he is 21. Though all of this is done, in the credits it says that Michael is 23 years old, which he is not. Someone didn't read the script and add years correctly!
This is actually a very common mistake! Michael was 8 when he murdered his sister then he escaped the mental institute 15 years later! That would make him 23!
Blown loose by the wind, the rain gutter suddenly swings down and
SMASHES through the window with a THUNDERING CRASH of broken glass.
Loomis jumps back, reaches into his coat pocket and draws a .357
magnum revolver.
Brackett stares at him. Loomis sees Brackett's reaction and slowly
reholsters the revolver.
LOOMIS
(looks at Brackett)
I suppose I do seem a bit sinister
for a doctor.
BRACKETT
Looks like to me you're just plain
scared.
LOOMIS
I am.
(he glances around
the bedroom)
I met his fifteen years ago. I was
told there was nothing left, no
conscience, no reason, no
understanding, in even the most
rudimentary sense, of life or death
or right or wrong. I met this six-
year-old boy with a blank, cold
emotionless face and the blackest
of eyes, the Devil's eyes. I spent
eight years trying to reach him and
another seven trying to keep him
locked away when I realized what
was living behind that boy's eyes
was purely and simply...evil.
Using a newly invented system called "math", we see that 6+15=21.
Taken directly from the "Halloween" script.
Although Michael was supposed to be 6 when he went on his murderous spree, it doesn't say that Dr. Loomis saw him when he was 6. Dr. Loomis doesn't say how old he was at that time. You also have to bear in mind that characters can be close to a birthday at the age given and that sometimes saying a number of years can mean 'almost 8 years' or '8+ years'. Added together, a couple of things like that can seem like inaccurate adding up!!!