I was adopted. I've known, forever. It was no biggie as I grew up because my parents couched it as me being chosen. My birth mother was always praised for her selfless sacrifice. So if anything, it was a point of pride. I knew other adoptees who weren't told and found out resented it. Always tell your kids.

https://onlysky.media/pzuckerman/if-you-know-someone-is-adopted-tell-them/