Metabolism differs primarily in four distinct stages of life. From infancy to age 1 metabolism accelerates to 50% above the rate of an adult. From age 1 to 20, metabolism slows by 3% per year. From age 20 to 60 it holds steady (although people gain an average of 1.5 lbs per year in adulthood). At age 60 there is a 20% decline in metabolic rate by age 95.
Read What We Think We Know About Metabolism May Be Wrong from the NY Times.
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When controlling for other factors (like age, weight, etc.) a paper found that there is no real difference between the metabolic rates of men and women.