This always kinda annoyed me with Spike and how the show creators went with him. Fluttershy made it clear that he was a "baby" in S1E1 but he does not really portray that. I think when they say "baby" they are talking about physical time and not mental or intellectual ability which is why I guess he still would fit that description. It seems pretty clear that dragons live for at least centuries in the MLP universe. With that said, I kinda wish Spike had changed physically somewhat throughout their journey up until now. To me, it would make sense that a dragon grows physically in a reasonable amount of time, but lives in adulthood for centuries or whatever. Or maybe in the FIM world not much time has passed at all. *shrug*
Considering that Seasons 1-3 took place over the span of less than a year, it's not really strange that Spike remains unchanged.
I don't remember a time reference ever given. Am I being stupid and forgetting something? Doesn't really make it the only way though. It might just accelerate the process.
The first episode of Season 4 is implied to be the first Summer Sun Celebration since the one in Season 1. Other than that, no time references were given.
At the end of S4 Ep2, Celestia talks about how the Summer Sun Celebration no longer celebrates the defeat of Nightmare Moon, but the return of Luna. While it is not specifically stated that it is the first Summer Sun Celebration since Luna's return, it is heavily implied.
