ReluctantBroney

My personal head cannon is that their ages run from 18 (Rainbow Dash probably being the youngest) to early to mid 20s at the latest (with Fluttershy being the oldest). With the general body type shown being for late teens and up. I will discuss these outliers first. In the Cutie Mark Chronicles Rainbow Dash and Fluttershy are both enrolled in some sort of summer flight school, this would at first seem to indicate that they are the same age more or less. However fluttershy seems to have a body type that I would classify as being a teen body type (its taller while not being filled out, seems gangly and awkward (much like a teen after a growth spurt) and is also very similar to Big Mac in this episode who is supposed to be older then at least the majority of the Mane Six) while Rainbow Dash seems to be smaller then the average pony in the flight school (while sharing their Child type bodies). Considering that Rainbow Dash is a very strong flyer and Fluttershy is a very weak flyer I find it reasonable to suggest that Rainbow Dash could have been skipped forward a couple of years ahead and that Fluttershy is being held back (I partially base this on my own experience with county level windbands and orchestras, often if you were very good you could progress upwards earlier then expected while if you were less accomplished you may stay at a level longer then expected (i.e. The Highschool Windband and Orchestra had a few exceptional 9 and 10 year olds while having a few 16+ members who did not have the ability to progress further)). Based on this I would say that Rainbow Dash in this episode would be 8-10, the average flight school pony would be 10-12 and Fluttershy would be 13 or 14. The fact they are friends would make sense to me as they were both being bullied (which would possibly drive them together simply because no one else will hang out with them) and I could imagine that the events we see in the episode are not the first time that Rainbow Dash has stuck up for the older but much more socially vunerable Fluttershy. Now the reason I assume that Rainbow Dash is about 17-18 at the start of season 1 is that this is the age that most countries allow you to join the military (and the Wonderbolts are definitly part of the military), this would put Fluttershy as being 21 at the youngest and 24 at the oldest with I'd say an age of about 22-23 being more likerly. I appologise for the huge wall of text but because I use these two ponies to work out roughly how old the other ponies are I wanted to make clear why I thought they were this age. Then based off of Chronicles I would put Applejack and Rareity as being about 20-21as of the start of season 1 (as they look to me somewhat older then Twilight, Pinkie Pie and Rainbow Dash but do not yet have the teen body type) with Pinky Pie being 19-20 and Twilight being about 18-19. However this age does clash with what Fluttershy said to Pinkie Pie in Griffon the Brush Off where she says she is a year older then Pinky Pie which would make her 21-22 at the start of season 1. This could in my opinion be because either the two writers/crews did not check for continuity on this issue (i.e. the boring answer) or that Pinkie Pie was later to develop or is simply smaller then the other ponies but due to how the bodies are laid out this isn't obvious due to design limitations (I refer to Winter Wrap Up and Applejack's comment on Twilight being strong for such a little pony (though this is itself unclear if she meant to imply she was strong compared to the larger male ponies shown pushing plows (who are bigger) or if she meant compared to other female ponies (which would be odd as they are all the same size)).