Background Pony #F331

If we're really getting technical, we don't even know that the CMC are all the same age. We only have what is implied by the show…



Spike was hatched by Twilight's magic surge which was in turn influences by the Sonic Rainboom.



Applejack saw the same Rainboom when she got her cutie mark in Manehattan.



It is implied that AJ's parents were already dead when she left for Manehattan, therefore Apple Bloom must have been alive… assuming Pear Butter is Bloom's biological mother.



By extrapolation, the CMC are all likely older than Spike, at least a little bit.



It appears that it takes more than just a cutie mark to be considered adults in pony society, otherwise Diamond Tiara would have been treated like an adult from her first appearance. It has been likened to more of a sign of puberty, and like Spike's recent molt, it is more of a symbol that the character is growing up, not that they are finished doing so. There are more implications that Smolder is considered a youth instead of an adult as well, and she has "seen her fair share of molts" and the impression is certainly given that she is a bit older than Spike. Heck, there has not been a dragon featured yet who has been younger than Spike, other than younger versions of Spike himself.



So… yeah. Whatever this all means in the grand scheme of things.