Our son was seven on this trip (December 2014) and he and one other boy (8) were by far the youngest (next youngest was 10) but he had a wonderful time. This was our fifth
ABD so he knows the drill and hung with all the other juniors. We also had a lot teens on this trip and the guides (Laura and Francesc--two of the best) even set up activities especially for them. My advice is don't worry about the ages of the children. All of the ABDs attract really wonderful children and on all of our trips the groups have gotten along well. My son is always the youngest but that's never stopped him from making friends. The times where there has been someone close to his age, he has not really hung out with that child. He generally prefers hanging with the older kids (they spoil him). On this trip, since it's in large cities, there are not as many breakaway times for the Juniors so I think your daughter will be fine. This was my son's favorite trip. He loved seeing the Eiffel Tower and we are making a special stop in London next year because he enjoyed it so much.