I am a retired socworker (30 yrs experience - retired 09) The answer to your query as far as I am concerned is asap. There should be no secrets that are going to come out in later life and cause huge difficulties for the person concerned. This was how thing used to be of course, and everyone thought it was the "best way"
Any child being placed for permanency by the LA should have a life story book (but they often don't have them) and if not, one should be made, with pictures of the child's birth parents, other birth relatives, houses they used to live in, the same for anyone with whom the child was fostered and building up to his/her permanent placement. The book should be like a picture book for young children, and by the time they are say 1 yr - 18 mths and can enjoy picture books, the life Story Book can be read along with the others, maybe birth mother being referred to as "tummy mommy" or something similar. The story can be told in simple language long before the child really understands, and can be brought out frequently and the child can point out "first mummy/daddy" whoever, and treated like a story book. Obviously as the child gets older, it can be shared between you and it is of course the child's possession and he/she only shares it with people he/she wants to.
Whatever the situation - I think it's a case of early as possible, told in a sensitive manner and allowing/encouraging the child to ask (and get answers) to any question that are raised. Sometimes we think children want more information than they actually do, and we should give information at age-appropriate levels.
Sorry I cannot evidence any of this, which I know is what you need, but I am sure you will find the evidence as part of your research.
Good luck with your social work career - I enjoyed every single day of my job and enjoyed friendships with colleagues (some of whom I am still in touch with) and consider myself really fortunate to have been in that position.