What is a safe age to let my kids share a bed?

My kids 4 and 16 months. Neither sleep well in their own beds. We started letting the youngest fall asleep in her sister’s bed then move her to her crib. We noticed that both sleep very well together but the baby is too young for us to feel she is safe sleeping with the 4 year old. Our main concern is our 4 year old rolling on top of her.

So what would be a good age for letting them share a bed? When the baby is 2?

We are not going to make them share a bed but rather let them if they want to. We were thinking 2 mattresses on the floor and let them crawl to where they want to be.