It would have been an interesting thing to see the CMC and Spike physically mature a bit. It's partly why people like Harry Potter. For many of the audience it would be fun to see them grow up alongside them.
I always headcanoned that the School of Friendship was for ages 11 to 17. At 11 a foal is now expected to have more responsibilities and to prepare for adulthood at 18. They'll therefore be often apprenticed at this point, like how Rumble was seen cooking with Gordom Ramsey pony in The Last Problem.
I always felt like with Cozy even if she were a little kid she probably was at least 12 to be able to be a junior assistant. And to have even older students listen to her.
Of course I doubt the show has this interpretation but that's how I imagine it.
Headcanon in Season 8 and 9: The CMC are 12.
Rumble is 13.
Spike is now 14.
Those two older foals in that growing up episode are also 14.
And the Student Six are 16.