Ok so you know how in MLP Twilight gets all worried in lesson zero and worries that she’ll be sent back to “magic kindergarten”.
Well I don’t think she’s actually talking about kindergarten.
We see what a 5 year old looks like with Swetiebell here and that’s very different from Twilight’s class mates who look to be the CMC’s age. We also know that Twilight and her friends were all about the CMC’s ages when they got their cutiemarks. Thus I think that the term magic kindergarten is actually just slang to refer to the first year magic class and not actually a kindergarten.
I think this also makes since based on how magic develops, we know that Swetiebell’s age group is just developing their magic so I think that’s probably the age Clestia’s school starts accepting at since you could know if some pony was actually talented.