Your metabolic rates at birth, adolescence, or your teen years are probably drastically different than your rate now if you're past 30. Is your average body temperature way less than 98.7 now?
Body volume scales roughly with the third power of height, while body surface scales roughly with the second power. Therefore, it is possible for the body temperature to stay constant if the basic metabolic rate (i.e., heat generation) per unit of volume decreases between birth and adulthood.
Besides, the human body has several ways of regulating core body temperature: sweating, restricting blood flow to extremities, goose bumbs, and putting on different clothes.
All good points. Do we have measured values for metabolic rate per unit volume as a function of age? This does seem plausible -- building a new body should take more stuff than maintaining an old body.
But I wonder if this translates into different adults saying that they have fast or slow metabolism as an explanation for their weight.
Perhaps I just don't understand what "metabolic rate" means. My body is a metabolic zero-sum game, unless I'm gaining or losing weight. What comes in as food goes out as heat or mechanical work. I suppose a higher heat output could be achieved at constant temperature by sweating more. A larger person (more skin) could of course generate more heat, but the energy has to come from eating more food or being less active.
Do you simply mean that I was gaining weight at birth, adolescence, and teen years? I could agree with that.
"What comes in as food goes out as heat or mechanical work." Or out as undigested food waste. Don't have an answer for you on metabolic rate, just adding the third way food calories can leave your body.
Besides, the human body has several ways of regulating core body temperature: sweating, restricting blood flow to extremities, goose bumbs, and putting on different clothes.