I have almost no <20 memories and I'm barely 40.
In fact when I started a dream diary for a personal experiment, I'm in highschool in most of them - literally, as the dreams take place in the building.
I went through the lucid dreaming rabbit hole when I was 17, I recently found the diary from back then and was curious to see how different the dreams are now.
Myself, I remember maybe one or two things per year from 4+ onwards if I focus; flashes of images. This gradually ramps up after ~12yo, and I can actually say I remember some events from around 16+ well enough to describe them and place in rich context. Properly detailed memory? That starts for me somewhere after 20. I'm 36.