R.S.
My parents told my sister she was adopted when she was 6 or 7 and all hell broke loose...she was never the same child again. They rowed a tough road with her after that...the "you are not my mother" card came out a lot.
She had some attachment issues when we got her...she was already a year old and had a very tough first year. She was pretty well bonded with my mom and working on getting closer to my dad (she feared all men), but it was like it unleashed a huge wave of abandonment issues and she did not want to reattach herself to anyone.
It was a very hard 10 years on my parents...she moved out at 16 and is still floundering.
After watching what my sister went through I am all about never telling a child they are adopted...in some ways i think the old closed sealed adoption process might have been better for some children.
Just my experience with my sister...maybe others have had better experiences.