It took quite a while before people started noticing that (non-avian) dinosaur growth didn't quite work the way it had been supposed. In fact, it seems to have taken quite a while before people permitted the notion of growth to enter their studies in the first place, which is how we got stuck with Brachyceratops and cheneosaurs (we would have all been better off without Procheneosaurus, but that is another story). By the 1970s, researchers had warmed to the idea that dinosaurs might change radically during growth. By the 1990s, suggestions that certain horned dinosaurs such as Brachyceratops and Monoclonius were really younger examples of other horned dinosaurs were at large. There was a hint of things to come in the punchline to the horned dinosaur story, which was that the full suite of horns and other excrescences did not appear until late in growth, but the implications were not yet clear.
It was about ten years ago or so that the rumblings started making it to the conferences. At the 2007 Society of Vertebrate Paleontology annual meeting, Jack Horner et al. presented an abstract concerning their findings on pachycephalosaurids, namely Dracorex was Stygimoloch was Pachycephalosaurus, because for reasons that remain mysterious Pachycephalosaurus liked to style in spikes as a kid but dropped them as it matured. The work was formally published in 2009, and can be read for free. More followed. Anatotitan represented old large examples of Edmontosaurus annectens. Most controversial, of course, was the "Toroceratops" hypothesis, that Torosaurus was just what Triceratops turned into as it grew up.
The issue of dinosaur growth comes up again because of a review paper by David Hone et al., which summarizes the various anatomical markers people have used to argue for a given specimen being at a certain growth stage (various fusions in the skeleton, bone texture, size, appearance of features thought to indicate reproductive maturity, etc.) and tackles how various growth stages have been defined. You should definitely read the paper if you are interested in this topic (again, it's free), and two of the authors have put out additional statements.
At the heart of the controversy is how we define things like "juvenile", "subadult", and "adult". When we use these terms, there are certain assumptions attached to them, but not everybody shares the same assumptions, and not everybody clearly states what they mean when they use them. This is a particular problem when dealing with a group of animals with growth histories that do not necessarily match up with the categories we're trying to put them in. Essentially, the more we look, the fewer dinosaur specimens turn out to fit our criteria for adulthood, indicating that reproductive maturity occurred before skeletal maturity. Once you've got "subadults" doing most of the breeding, you've got some important implications for selection pressures, ecology, classification, and so forth, and you're left with the possibility that dinosaurs may have more or less abandoned "adulthood" as we typically understand it. This would be pretty darn significant. Take the Morrison Formation, for example. What if, instead of the "functional unit" of Apatosaurus ajax or Diplodocus carnegii or Camarasaurus supremus being the giant sauropods we know from museum mounts, the basic reproductive units of these species were much younger and smaller individuals? For one thing, you've greatly reduced the amount of food needed to support viable populations.
Inevitably, my thoughts return to "Toroceratops". One of the major issues, at least in my consideration, is a question of selective pressures. Let us assume that "Toroceratops" is accurate, and let us further assume that reproduction began during the Triceratops stage. I make this assumption in consideration of the great numerical disparity between Triceratops and Torosaurus specimens. It would seem to be a poor way to run a large vertebrate taxon to have so many individuals keel over just before reaching reproductive maturity. Under these two assumptions, my original question was what selective pressures would be responsible for turning Triceratops into Torosaurus if it was already breeding quite happily (we presume) as Triceratops and so few individuals seemed to be living long enough to make that change. In other words, with a perfectly functional Triceratops, what is the advantage of going Torosaurus? But what if this was asking the question from the wrong direction? What if instead of there being selective pressure to attain the Torosaurus form, there was pressure to prolong the Triceratops form, making the Torosaurus form a relic of an earlier stage of the Triceratops line? In other words, might Torosaurus not represent the "adult" of Triceratops, but rather something like a geriatric form that still appeared late in growth because there was limited selective pressure for or against it? One implication of this idea is that there should be geologically older taxa that shifted from Triceratops-like morphologies to Torosaurus-like morphologies earlier in growth.
Campione, N. E., and D. C. Evans. 2011. Cranial growth and variation in edmontosaurs (Dinosauria: Hadrosauridae): implications for latest Cretaceous megaherbivore diversity in North America. PLoS ONE 6(9):e25186.
Hone, D. W. E., A. A. Farke, and M. J. Wedel. 2016. Ontogeny and the fossil record: what, if anything, is an adult dinosaur? Biology Letters 12(2).
Horner, J. R. and M. B. Goodwin. 2009. Extreme cranial ontogeny in the Upper Cretaceous dinosaur Pachycephalosaurus. PLoS ONE 4(10):e7626.