It's entirely normal and extremely common. It varies of course, but generally memories mutate to some degree upon each revisit. It's not so much playing back a "mental video" as it is picking bits that stood out and tossing them back into a story that makes sense to you at that particular moment.
As to why this happened more immediately and to a more extreme degree when you were younger, I would guess it's an artifact of growing up and the brain changing, making, pruning connections during that time. I remember next to nothing from when I was young other than some bits related to school. I also had some wild no-way-they-could-be-true memories related to having a kooky kid-imagination.