Flutershy is oldest. Next applejack. After her is Rainbow. After Rainbow is Ratriy. After her is Pinkie, and the youngest is Twight.

fluttershy is defiantly the oldest i dont get why people think rarity is if anything she is the second oldest lol fluttershy looks older then all of them and we saw applejack a a teenager and she as the size of fluttershy when a teenager so we know that she was a teenager when they were fillies it isnt hard to see plus the implications this one is pretty obvious rarity is the not the oldest fluttershy is plus they say rarity is mature i dont really see that she cries over the dumbest things in immature crushes always tries to make everything about herself hence forth is way shorter than flutters and overall can be very egotistical

They’re obviously all around the same age, and Fluttershy was exaggerating when she said she was a year older than Pinkie. She’s still significantly older, but not enough that they wouldn’t be in the same grade together

Rarity: 16 1/2  
Fluttershy: 16  
Rainbow Dash: 15 3/4  
Applejack: 15 1/2  
Pinkie Pie: 15  
Twilight Sparkle: 14 11/12

Okay. One more comment. If we are going by “Word of God” that Flutters is 22 the that would stand to reason Pinkie was 21 at least until Party of One when she became 22. How old do you think Cheerilie is? Probably the same as Rarity give or take a year. This makes Rarity very likely the second or third eldest.  
Since AJ was the last in her class for a Cutie Mark, and Rainbow was first even though they got them at the same time, this stands to reason that AJ is older than RD. Twilight had just begun Kindergarden, since every other pony but Pinkie was shown to have been old enough for school (or camp) ten I’d say Twilight is the youngest.

@Background Pony  
That’s an amazing point! Lets say Twilight was 5 or even 6 in human years. If we say each pony is almost a year away from another pony then we can say that whoever was the oldest was about 10. So this means ponies can get their marks anywhere between 5 and 10 or whatever. Unless Twilight was early.If we think of the CMC ages then thy are probably around 10 or maybe even 11 years old

In my opinion (From oldest to youngest)
 
1 Fluttershy  
2 Rarity  
3 Pinkie Pie  
4 Applejack  
5 Rainbow Dash  
6 Twilight
 
Twi and Rainbow may switch but I’d say hey are all around the same age. Just by the way they behave to each other and other logical guesses

Twilight attended kindergarten at the time. She is surely the youngest.

Since ponies live hundreds of years, the mane 6 might not looks much older till their fifties or something, but this is all just a guess.

Addendum: Summer Sun Celebration - Summer solstice, maybe?  
Applebuck season- end of summer/ early autumn (Typical harvest season)  
winter wrap up- winter (duh)  
Call of the cutie- spring- cast is now 1 year older than series start  
fall weather friends- autumn  
next few episodes- spring/ summer- cast is now a total of 2 years older than series start  
Return of Harmony stated to be a year after Best night ever- 3 years since series start  
lesson zero- spring/ summer  
luna eclipsed- late autumn  
sisterhooves social- spring- 4 years since series start  
hearth’s warming eve- winter  
last roundup- summer- 5 years  
hearts and hooves- spring, most likely- 6 years  
dragon quest- autumn  
canterlot wedding- spring- 7 years
 
Currently, Rarity is 22, Twilight 24, RD 25, AJ/PP 28, FS 29, and Spike is 19 (Dragons just don’t age normally.)
 
Thus, Sparity is a perfectly reasonable ship.

Rarity talking with a babyish lisp= 3 years old (and helping her mother with sewing, not owning her own business)  
Twilight starting magic kindergarten= 5 years old  
Rainbow Dash maybe a year older than twilight= 6 years old  
Applejack allowed to leave home unattended= 9 years old  
Fluttershy hit a growth spurt= 10  
Pinkie Pie is only a year younger than Fluttershy= 9
 
Fluttershy is stated by Lauren Faust to be 22 when the series starts, meaning 12 years have passed, making Rarity 15, Twilight 17, RD 18, AJ 21, And Pinkie Pie 21. Side note, Spike is 12.

Twilight is the youngest?  

It occurs to me that if Applejack was the last in her class to get her cutie mark and RD was the first (Call Of The Cutie), and they both got theirs at the same time (Cutie Mark Chronicles), then Dash being older also takes care of THAT little inconsistency.

fluttershy is a year older than pinkie, she said so in an episode.

The only thing that indicates that they other are older than Pinkie Pie is speculation. Going by how they act says little, since it is an act.
 
Rarity has never said that she particularly likes Ponyville, has little tying her to the place, and would get more business from a higher class of client while being around the social life she so desires in Canterlot.
 
I was under the impression that Sweet and Elite was Rarity’s first solo trip to Canterlot. The more I think about it, the more sense it makes.

Maybe they only used that filly size once to illustrate Fluttershy’s age? And do you think it’s at all possible that she’s bad at flying because she’s afraid of heights? (which has been confirmed)

Okay, if the animators and writers only used that filly size once, then there must have been a reason to use it.  
They must have been trying to convey something, such as her being older.
 
Simplest explanation

@Advance
 
Another assumption. Do you think they change that much in 1 year? (and that’s already ignoring that they have only used that filly size once). The flashback already provides a simple and logical explanation, she was going through a growth spurt, which explains her clumsiness.
 
@Sjogre
 
Because, looking past speculations, it’s not hard to think that any of the others is more than 1 year older than Pinkie.
 
And Rarity has said that she likes the life in Canterlot (though not so much since S&E), but she hasn’t said that she wants to move there. Like AJ, she likes her life in Ponyville.

She was able to make dresses prior to getting her cutie mark. She simply didn’t like them until she discovered gems
 
Her cutie mark is related to gems, rather than dresses

Or maybe she’s good at making dresses because it’s her special talent?

Rarity didn’t own her own business in cutie mark chronicles but she was working on it
 
She was able to make dresses very young so she had probably been learning that for awhile and started on being a fashionista afterwards since she had her heart set on that.  
It’s nothing definitive but it suggests she was one of the older ones

FYI, Fluttershy says she’s a year older than Pinkie Pie. So however the age-structure is built: its canon that Fluttershy should be higher up on it than Pinkie.

Why does being only a year older imply that Fluttershy is among the youngest? What implies that the others are older than her or Pinkie?
 
You might as well say that since Rarity and Applejack are the only ones that haven’t moved out of the town that they were born in, they’re the youngest two. Well, Applejack wants to stick around, but Rarity wants to move to Canterlot, so she obviously hasn’t been on her own for long.

@Robomorons
 
No. By the looks of it, Twilight is in her early college years. This could be anywhere from 18 to her mid twenties.

@BP  
That seems like the simplest and most logical option. It’s obvious that the writers put it in there for continuity.