I told my dd at 6, and tbh wish I hadn't.
I get so many question, at inappropriate times tbh. I am happy to talk about it, but when I chose to, not having it thrust on me.
She said some things that really upset me, like "Do I have another Nanna then", do I have another grandad etc etc.
I'm afraid I snapped at her and said NO, Nanna was my mum etc and that I don't have another mum and dad....
I didn't think it through properly before I told her, it came out of a similar conversation as yours, asking about my birth etc..
I hadn't realised it would effect her, I hadn't thought of how she would feel.
I remember someone asking me last year if I would ever try to trace down my biological family and I said no, not interested. They said I was being selfish as I was denying my DD of learning about her history... I was shocked and am still trying to comes to terms with that..
My problem is that I am not looking for anything. I had the best childhood, the best family life I could ever had had, I am not missing anything. I am thankfull I was adopted. Unfortunately both my mum and dad have now passed away.
But for DD it's different. She hasn't got any grandparents and is an only child...
Anyway sorry went way off track..
I would wait a few years until she has left to be sensitive to your feeling.. I know at 6 my DD wasn't capable of that.
To be fair she is now 7 and is much better.