We have all been told that after 20 years of age your metabolism starts to slow down, and you eventually gain weight. That men have a faster metabolism then women. That women 
have a harder time losing weight as opposed to men especially after menopause. Now after a new publication in “Science” these assumptions seem a little misguided. Data from 6,500 people ranging from 8 days to 95 years of age, allowed researchers to divide a lifetime into 4 distinct periods of metabolism. The study involved 80 co-authors who shared data along with information from 6 labs using data from the past 40 years. Calories were measured by what is considered the “Gold Standard”. This measures the number of calories burned by tracking the amount of carbon dioxide expelled after daily activities. All factors were taken into consideration- height, weight, percentage of body fat. 1) Infancy -metabolic rate is 50% higher than adults, 2) Age 1 to 20 metabolism slows about 3% a year, 3) 20 to 60 it holds steady, 4) after 60 declines about 0.7% per year. They found no difference between men and women. Although the metabolic rate of individuals varies, the general pattern holds true for life. The results dispute the notion that there is a constant rate of expenditure per pound. Metabolic rate is shown in these studies to be dependent on age.
https://www.sciencedaily.com/releases/2021/08/210812145028.htm