I can't remember who said this already, but I watched a lecture on this a few years ago. Talking about for instance the different triceratops breeds, and how they could mostly be juveniles, but every scientist was too fond of "finding a new dino" they forgot to ask simple questions. The guy said that if we could examine the fossilized bone marrow structure it may indicate the age, but they don't exactly let you just start cutting into fossils do they?