ELI5: Why does your back hurt when you sit on the floor as an adult?

When I was a kid, we used to sit criss-cross on the floor all the time at school with no problem. My husband and I put a ping pong table together (the wheels were kinda complicated), sitting down on the ground in the garage. Our backs started to hurt (age 29 and 32).

I always remember as kids..the adults saying stuff like “you’re young, you can sit on the floor”… is this why? or them sitting in chairs and the kids being on the ground at certain events, like at the beach