I'd disagree. If you follow that the dragon fluttershy chewed out was going to nap for 100 years, then it shows dragons having an unnaturally long lifespan. I kind of took the last episode to show that dragons were supposed to grow slowly as they accumulated more possessions, over several centuries. Spike went nuts and tried to steal a town, and ended up looking far more deformed than the other dragons we've seen so far, I took that to mean he was not aging naturally at all, but it an accelerated pace unusual for his species. I think the other two dragons support that too. They were definitely greedy jerks, but they weren't actively destroying towns, and they seemed to only take gold and jewels, which spike has naturally lusted after since the first few episodes. They seemed to have a much better handle on their greed.