From National Geographic News
There is a theory that as many as 1/3 of the known dinosaur species may not have been different species but rather they are different stages of the same animal. The video on page 2 cites the triceratops and a modern day bird, the hornbill, as examples of creatures with age related differences. The triceratop's horns grew in curved backwards and later turn to the front in the adult that most of us are familiar with. They give some reasons for this but unfortunately get too much into anthropomorphism in their reasoning. Basically the backwards curving horns would have prevented the juveniles from competing for mates but would have still been good for protection or grazing.
We will have to wait to see how this plays out, but it will give a good framework for future understanding of some of the world's coolest creatures. (Tho it might be a bad thing if your favorite dino wasn't its own species after all)