I think children should know as soon as they are able to understand the concept. There are many wonderful books which deal with the subject and if you talk to them from an early age, the child will accept it much easier than if he/she finds out later on in life.
As an adopted person I'm grateful that I was told when I was little (so little I don't remember ever not knowing). It meant there was never a drama about it. It was just a fact of life. I have heard stories of people who only found out they were adopted in their teens or as adults and that sounds pretty traumatic so I would say preschool age would be about right.
I was determined that both my adopted children would know as soon as possible. Unfortunately my three-year-od son had an auditory processing difficulty and had just had a poor hospitalisation experience. He ran screaming from the room shouting, "I don't want to be a doctor!"
My daughter had no such problems, and her precocious 4 year old talks matter of factly about her Mum being adopted. She recently remarked, "Mummy's mother might have grandchildtren by now." I didn't reply with the obvious.