Your Metabolism Isn't Slowing Because of Your Age...
Remember when you were 25 and could eat anything you wanted but not gain any weight?
Then, the 30's hit and that changed a bit.
Now that you're in your 40's, 50's, 60's+, you can just look at food and gain weight.
We've been conditioned to believe that our metabolism slows down as we age.
New studies are showing that this isn't true.
According to studies done by Science, your metabolic rate really doesn't slow down between 20-60 years old, only declining by a little under 1% a year after 60 (until you get to 90 where it substantially changes).
Your metabolism is what allows you to stay alive. It is comprised of your Basal Metabolic Rate (BMR or RMR (resting metabolic rate) which is the amount of calories you burn just being alive including breathing and maintaining basic function. We then have Total Daily Energy Expenditure (TDEE or can also be called TEE) which combines your BMR with any activity you do throughout the day from fidgeting to working out, alongside dietary thermogenesis, or the calories it takes to digest your food.
According to an article from Harvard, "the study authors were interested in effects of physical activity and tissue-specific metabolism (the idea that some organs, such as the brain and liver, use more energy than other organs, and constitute a higher percentage of body weight in younger individuals) across the lifespan. Through various modeling scenarios, they determined that age-related changes in physical activity level and tissue-specific metabolism contribute to TEE across different ages; in particular, elevated tissue-specific metabolism in early life may be related to growth or development, while reduced energy expenditure in later life may reflect organ-level metabolic decline" (source).
So, what does all this mean?
It means that while your metabolism doesn't necessarily slow down as you age based on the factor of aging, you do. As we get older, we slow down. We tend to move less in general, then add in working more sedentary jobs, relaxing after work, spending more time sitting around or sleeping and performing less daily activity. This, in turn, does cause you to burn less calories throughout the day and down regulate your metabolism. While your metabolism itself doesn't speak for how much food you eat, people are generally consuming too many overall calories for their individual metabolisms leading to increased weight gain as they get older from not adjusting food intake to activity level/down regulation of the metabolic rate from overall activity levels.
If you're trying to combat weight gain as you age, ensure that you get adequate physical activity during the week, eat a balanced diet with plenty of protein and nutrient dense foods, get plenty of sleep and reduce stress levels.