To take just one example…
We lose muscle mass from the age of 30 and so face a decreasing metabolism. This means as we age we face a losing battle even before we get out of bed.
When James Cracknell was taking part in the Oxford & Cambridge Boat race he was more than 20 years older than his team mates.
As part of the training they all had their metabolic rate tested. This basically told them how many calories they needed at rest per day to simply survive…how many calories they’d burn just sat on he sofa doing nothing all day.