I have vivid memories of things from when I was little more than a toddler but have to think about what I did yesterday.
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I have often pondered this. My late father-in-law remembered in his 90s the names of all of the students in his 6th grade class. He remembered in vivid detail many, many events from his life. I suspect that we retain those childhood memories so well because they were stored when all of the equipment was newer and had not been battered much. I put that facetiously, but I am sure there is some biochemical and neurological analogy.
Always astounding is the way a particular odor can take you back to a moment far, far in the past. For example, whenever I smell an orange, I think of childhood Christmases because we always had an orange in our stockings.
I know that smell is pretty closely tied to memory. The sight of an old shotgun shell laying on the ground brings the smell back in a big hurry.