This supports this conclusion. AJ and RD are the youngest.

@Dave Berrow  
I really need to point this out - Fluttershy is actually depicted the same way Applejack is in Where the Apple Lies during the flashback - kind of lanky. If anything, Fluttershy being the poor flier she was, was held back.

More likely it’s all just a bunch of gags and irrelevance.  
Lets look Cheerilee. She is most likely in the same class as Rarity is in ep. “The Cutie Mark Chronicles” where Rarity does the costumes for the play. But in “Pinkie Pride” we see that Cheerilee is as old as Pinkie. We also know that FShy is one year older than Pinkie Pie, but around the same age as RDash since they’ve shared the same flight class and generally grew up together. Since Derpy is (a bit older but almost) the same age as Dash we can assume that Dash is the same age as Rarity, because Derpy is the same age as Cheerilee in “Pinkie Pride” which in turns corresponds to Cheerilee being the same age as Rarity in “The Cutie Mark Chronicles”… We’ll get back to this part latter.  
So only Twilight and AJ remain unknown thus far. Lets start with AJ. Take note that the flashback in “The Mane Attraction” takes place before “Where The Apple Lies” due to AJ being younger there. AJ writes to Rara but it doesn’t mean that Apple Bloom is born right after the camp. Okay, now I need to mention that AJ Apple Bloom isn’t around at the time of “Where The Apple Lies”, but she appears to be the same age the other cmc are. So we can suppose that AJ is somewhat the same age as Rarity is, since she(AJ) is probably as grown as Rarity is in “For Whom The Sweetie Belle Toils” 5 years from “Where The Apple Lies”. And since Rarity is as old as Cheerily we can assume that AJ is too. But that would be some sort of a nonsense since ChL is clearly older.  
One more thing to consider. We now can assume that RD is the same age as Rarity, right? But we know from the episode “The Call of The Cutie” that Dash got her mark first in her class. While Apple Jack got her mark last in hers. So doesn’t that kind of makes AJ older than Rainbow Dash if they got it at the same time? Unless on average pegasuses are getting their marks latter than other ponies.  
Okay then, maybe we’ll get more luck with Twilight? Lets see, there are her childhood friends in “Pinkie Pride”, Minuette and Lyra, and they are shown the same age as ChL. So probably Twi is also the same age? But wait there also are the other two, Demon Hearts and Twinkleshine, but they are shown as adults. Seems like we have a problem here…
 
 
So you see, with all the cloning and swapping and the general way the show treats those things, it is all quite meaningless.  
I mean the age is more or less a variable shifting from episode to the episode to cover the topic and symbolism the show aims at. But it is nice of you to notice those things.  
Also this >>1441888

For Fluttershy, Lauren had said it was meant for Fluttershy to be tall, gangly, and extra clumsy for her age (much like herself).

@Grey Silver  
The Granny being younger later on I’d say was just the smaller budget of Season 1.

At least we know Pinkie&Rarity R much older than Dash & AJ  
Fluttershy is 1 year older than Pinkie , age of Twilight still a secret

I can’t tell what the argument is here.  
I see no connection between the pictures explaining how the conclusion is reached.

i had no idea how old Berry Punch was. I still don’t know her exact age but why she doesn’t look that much older in the present day & seems to blend on well with the rest is baffling.

Technically we don’t know at what point the camp AJ and RaRa were. In fact the only points we can place exactly from this set are the events of “CM Chronicles” as being the first in the timeline. The camp picture looks like it takes place before “Where the Apple Lies” (their proportions and amount of AJs freckles).
 
Also from the staff info from early on, we learnt that the CMCs were about 8-10 during the 1st season, so now we can assume they are 11-13, so Sweeties 5th BDay took place 3-5 years before “Elements of Harmony”
 
Also during “Where the Apple Lies” Granny looks younger than in “Cutie Mark Chronicles” (my personal theory is that Starlight unintentionally gave the timeline a concussion, thus plotholes)
 
Yet overall I do agree that if trying to arrange the Mane 6 in order by age, AJ would be the youngest or second youngest to Rainbow