I still have multiple distinct vivid memories from when I was between 18 months and 2 years old…memories I verified were real and accurate decades later with the people involved. I have some vague memories of even earlier, around 1.
Obviously if this is a “rule”, it’s flexible.

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What were they?

I think the earliest memory was when I was "Home Alone" and my queen was outside.

At under 18 months I drew on my grandma’s new fancy wallpaper….the getting dragged into the other room and the spanking I got is my first memory.
My second memory was while our house was being built, so I was under 2, and I climbed out on the exposed beams above the atrium, about 35 ft up and 30ft out on 10” wide beams…my mom held it together until I toddler walked back to solid ground then lost her shit. I was trouble.