Yes it is. Ever since "Rescue at midnight castle" have he been refered to as a baby dragon.



Even in the pilot of FIM he is refered to as a baby dragon by Twilight Sparkle herself. " You are a BABY dragon after all."



Also in Dragon Quest we get to see how teenage dragons look and they are much bigger and human like.



The only Spike in the MLP franchise which is not a baby is the G3 (he is over a 1000 years old) and he still is the same size.