system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do our energy levels diminish as we age?

Children have an abundance of energy, but as we get older we seem to lose those energy levels. 

Is there a specific factor (or set of factors) that cause humans to feel less energized as we get older?
gpt
Kids don't have more energy than adults.  Adults can outrun, out walk, and out perform in any energy demanding task there is.

What kids have is excitement and playfulness.  It is more of a psychological drive than anything else.  If an adult is truly motivated or excited about a sport or activity, you can see that adult perform it for hours and hours and hours; however a kid could not keep up to that pace.

The same is seen in adult animals. An adult wolf could track its prey for miles and miles and miles. However, adult animals do not partake in play activity, for it is a waste of energy, but crucial for young to develope their motor skills.