Personally, I was more surprised by the various groups of people who _don't_ have higher metabolic rates: > Children’s metabolic rates stay high until age 5, but the rate slowly begins to glide down until it plateaus around age 20. Interestingly, adult rates are stable until age 60, when they begin to decline. After age 90, humans use about 26% less energy daily, Pontzer says. > The study also found that pregnant wom…
This seems almost unbelievable to me. Almost everyone I know, with a handful of exceptions I can think of, had a dramatic change in what I can only say was metabolism at 30 declining steeply to 40. I can tell you personally my ability to lose weight when exercising daily has decreased significantly since 30. If it’s not metabolism, then what? Diet and exercise are almost identical.
Also to compound this your metabolism tends to adapt to whatever you are doing. Even if you are doing lots of exercise, your body does tends to become more efficient at doing that exercise, using less calories to do the same thing over time. The same is true of running on lower and lower calories, your body adapts to run on less calories so it actually helps to do something of a 'bulking' period every so often to keep your metabolism running at a higher rate and not letting it drop down to 'economy mode'.
If you change nothing from your 20s to 30s, keeping exactly the same calories in your diet and same amount of exercise then you will most likely start to gradually gain a calorie surplus from this loss of muscle and thus start gain some more fat from the excess calories. You'll basically need to reduce calories or increase exercise to compensate.
Also look into what you can do to help maintain/improve your natural testosterone production (better sleep, nutrients like zinc and magnesium etc) as this will help offset it to some degree (poor sleep alone will absolutely destroy your natural testosterone levels). Alternatively you can look into Testosterone replacement therapy, which seems to be quite common these days.