Processed foods, lack of exercise, Starbucks with a lot of sugar in them.
^^^
Watch a child for 15 minutes, if he/she is not sleeping and you have been wise enough not to turn on a screen, you will see that his/her movement in that time is equivalent to what adults do in a crossfit session.
In the words of someone who was in medicine, it is not our metabolism that slows down, but we adults who slow down our physical movements.