Eli5: why do babies sit with perfect posture when they learn to sit, but the older you get, the worse it gets?

Question

Babies sit straight up naturally. But the rest of us have to put effort in to having good posture, why?

In: 1304

Answer 1

Since I started weight lifting I am forced to have good posture, otherwise my muscles start aching almost immediately because the muscles are in an unnatural position. This makes the bad posture very uncomfortable.