Michael Myers' age

Rating: Null/10 (22 December 2004)

In the film, it was said that Michael was aged six when he murdered his sister - then later on in the film, somebody mentioned that he had come back to Haddonsfield after 15 years of being locked away in the hospital (which according to the credits at the start is fine because when he was 6 it was 1963, fifteen years later it is 1978). So how comes, when Michael comes back, he is now 23, two years older than he is supposed to be? 6 years old plus another 15 years would make him 21, not 23 as stated. Please correct me if I'm wrong, but I don't get where his age changed...? If anyone would like to tell me where I went wrong it would be much appreciated as I'm confused.