Understanding Metabolism: A Deep Dive into How It Works & Why It Changes With Age

Question
Hello

I am writing a report for my science class in High School. Junior. I am writing about metabolism, what is is, what it does, etc.  

I find a lot of information on metabolism; suprising because it apparently is very complicated.

I also learned that as we get older, and older, our metabolism slows down more and more. But I cannot find any answers as to why it slows down!
Have you ever read anywhere as to the why?

Here is a guess that I am writing for now;

Ever watch little kids? They do not stand still or sit still. They are always jumping around, moving around, etc. Parents constantly tell them to "sit still," and "stand still." From what I read - it APPEARS that our metabolism's "engine" is aerobic movement of our bodies, ie. people who constantly move, such as rocking their legs back and forth when they sit, etc. seem to have higher metabolism, thinner bodies, than the rest of us who - as taught by our parents - sit still and stand still. This is my own observation. Like I said, I cannot find anything on what makes metabolism go, and/or slow down.

Thanks for the help.

Answer
Hi Judy,
As you said Metabolism is a complicated subject. There are any number of reasons why metabolism slows down with age. Not all older inactive people get fat and many young active people do. Metabolism is basically the act of converting nutrients into useable energy for the body. Thyroid function, Pituitary Hormone, Insulin, Digestive disorders, heart disease, some brain diseases all contribute to metabolism. It is a very complex subject and we havent even discussed bio-chemical implications. I don't think that you can do a report that adequately explains what Metabolism is and does with any depth without writing a novel. Try to keep is a simple as possible. Save youself writers cramp and paper.