Harriet, you don't sound like a twat at all! I always take the position that its for the adoptee to decide as ultimately they had the least control over their adoption of anyone and anything they need to do to take control back is kinda fine by me (note I do say kinda
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My advice to tell and as early as possible is based on -
a) studies (admittedly on adopted children rather than children of adoptees but suspect its similar) show that those children who can never remember being told (ie they were told before they could even understand what it really meant) tend to process the fact of their adoption more readily than those who can remember "the moment".
b) the OP asked "do I tell my children I was adopted"? - so I gave my opinion. She isn't compelled to do it, if she doesn't feel able but I assumed from her question that she wanted to hear what others think becasue she isn;t as certain as you about what would be the right thing for her and wants to hear what otehrs experince is. You aren't obliged to either listen to our opinions or act on them and giving her an alternative view is I guess what the thread is about.
c) within the adoption community people see adoption as a perfectly normal run of the mill thing as its just a part of the facts of your life (not that the emoticons around it are run of the mill just that the process and reality of it is a normal part of their life not something very out of the ordinary). My Aunt (now in her 60's) was adopted and my son (nearly 6) was adopted so its been in my family for such a long time that I forget that people not in the inner circle (so to speak) tend to treat it as a strange and wondrous beast. By not sharing the facts with your children you are IMVHO placing them in the outer circle and not admitting them into the inner circle is sort of how I look at it.
I wouldn't be presumptuous enough to tell an adoptee this is what they must do and would be pretty pissed off with someone without any experience telling me what it is like being infertile. However as an adopter and someone who has an adopted Aunt who has not only children but also granchildren and even great grandchildren, I would hope that my experience gives me some insight into how it can work without much fuss. My experince is that when told fairly young, most children/grandchildren are disappointingly uninterested!