I also think that the foster parents should tell their children that they were adopted. The thing is, of course it will hurt, since it's not something you get to hear all the time. It is shocking to find out that those parents you loved and cared for aren't actually your real parents.
However, they accept it after some time. Of course the children will be hurt by this for some time, but eventually they will get along with the situation just as well as the foster parents do.
Of course that period of being irritated and shocked can last very long but, as anime_being_god said it right, it is up to the respective foster parents to know the right time to tell it, so that this period won't last very long.
Although I think that 14-17 already is a little late. At this age, the children might have already found out about their real origin. Either the children really ask their foster parents since they might look very differently or the more possiblly they find out in school. Because I guess that parents might talk about this with other parents and because it is known in school. So I guess that the children have already found out about it in such an age.
Additionally, young children can get along with such news better than one might believe. I think one should tell their children about that earlier.