Why is it okay for young children to sleep on stomach but not adults? i need some advice.

Flexibility: Babies should sleep on their backs until they learn to roll over well on their own. Children sleep in all sorts of positions, mainly because they are more flexible than adults. As we age, our back muscles are strained more when we sleep on our abdomen, and can cause backaches. Sleeping on your side or back, with a pillow under the knees, keeps your back in a more neutral position.