Metabolism a myth? Ummmm....no....you're largely responsible for your own metabolism. Of course, with age it will break down somewhat, but if you're conscientious about the amount of physical activity you get you won't balloon to twice your normal size if you maintain a particular diet.
I think we're using different ideas of metabolism here though, that might be the source of confusion.
I'm referring to the common use of the word "metabolism" as the base rate of burning energy, i.e. disregarding physical activity. That's what people mean when they say "my metabolism slowed down", they mean that with similar physical activity during the day than when they were young, these days they start accumulating weight.
With that definition, IMHO the statement of "I'm gaining weight because my metabolism slowed down" is mostly a myth. It's because of the lack of physical activity that you're gaining weight, not because your body inherently processes caloric intake differently.
rumborak