Ages

The pie sisters , I think are:Maud(oldest),Limestone(second oldest),and then the youngest are Pinkie(older twin)Marble(younger Twin)  
there’s a card out there that says Limestone is the oldest,but it also says there is another older Pie sister aside from Maud .

@Dex Stewart  
Limestone is officially the oldest.

@Magma_ERuptiOn  
I always considered Maud to be the oldest,since she wasn’t in Pinkie’s cutie mark backstory,I figured she was off in school studying rocks or whatnot.

@Magma_ERuptiOn  
When was this confirmed?

@Background Pony #C740  
I think it was a Big Jim Twitter answer, immediately after the episode.

Rarity is the same age as Cheerilee, since there in the same class, which would make her mid 20-early 30.
 
That or Cheerilee got held back alot

@Tumblr Feminism Is A joke  
The only proof of that is in Season 1, where the budget was so low they had to resort to recycling Trixie.

Headcanon, as of Season 1:
 
Twilight: 20  
Rainbow Dash: 21  
Applejack: 22  
Pinkie Pie: 22  
Rarity: 22  
Fluttershy: 23  
Apple Bloom: 10  
Scootaloo: 10  
Sweetie Belle: 10  
Spike: 10

@Wiimeiser  
Filly Cheerilee’s flashback appearance in Show Stoppers wasn’t a recycle,

Facts as we know them from the show:  
Fluttershy is a year older than Pinkie Pie. (Griffon the Brush-Off)  
Rainbow Dash was the first in her class to get a cutie mark. (Cutie Mark Chronicles)  
Applejack was the last in her class to get a cutie mark. (CMChron)  
Twilight Sparkle entered Celestia’s school at the time she got her cutie mark. (CMChron)  
All the Mane 6 except Applejack got their cutie marks at exactly the same time. Applejack got hers a short while later, when she got home. (CMChron)  
Rarity and Applejack both grew up in Ponyville, Rainbow Dash and Fluttershy grew up in Cloudsdale, Pinkie Pie grew up on a rock farm in the middle of nowhere, and Twilight Sparkle grew up in Canterlot. (Various)
 
From this we can deduce:  
For sure, Fluttershy is older than Pinkie Pie. This isn’t even a deduction, it’s stated fact.  
If Fluttershy and Rainbow Dash got their cutie marks at the same moment, and Rainbow Dash was the first in her class to get hers, Fluttershy must also be at least one year older than Rainbow Dash.  
If Applejack got hers last in her class, and she’s in the same town as Rarity, Rarity must be at least one class year younger than Applejack. Rainbow Dash, getting hers first in her class, must also be at least a year younger than Applejack.  
Pinkie Pie wasn’t in school, but is only one year younger than Fluttershy. She can’t be younger than Rainbow Dash, though she could be older than Rainbow Dash.
 
It then follows:  
-Fluttershy is the oldest.  
-Pinkie Pie is one year younger than Fluttershy.  
-Applejack might be the same age as Pinkie Pie, but cannot be older. She could be younger.  
-Rarity is younger than Applejack, but may be the same age as Rainbow Dash, younger than Rainbow Dash, or older than Rainbow Dash. But she cannot be older than Applejack, so she also cannot be older than Pinkie Pie.  
-Twilight Sparkle is the youngest.
 
So the order is:  
Fluttershy (oldest)  
Pinkie Pie  
Applejack (Odds are Applejack is younger than Pinkie Pie. She can’t be older, but there are more ways she could be younger than that they could be exactly the same age.)  
Rarity  
Rainbow Dash (Rainbow Dash is probably younger than Rarity since Dash was the first in her class to get a cutie mark, and Rarity was not, even though they got their cutie marks at the same time. But since they were in different classes that’s not conclusive. But it is very suggestive.)  
Twilight Sparkle

I tried to search for some information that would prove that all mane6 are at least young adults, so here it is what I got:
 
“S05E25 – Twilight Sparkle: Okay, so, um, hey, you want to race?  
Young Rainbow Dash: Um, that wouldn’t really be fair. I mean, you’re a full-grown pony, and… Wait. Are you an Alicorn?  
Twilight Sparkle: Come on, I thought you were fast!
 
S06E14 – Rainbow Dash: And it’s probably best keeping all the grown-ups on the sidelines. But… what are we supposed to do now?  
Rarity: I think I have an idea.
 
Ponyville Mysteries #2 – Don’t be scared, Sweetie Belle. We shall be fine! Rarity called. The grown-up ponies all hurried into the Forest, Twilight leading the way.
 
S07E24 – Starlight Glimmer: I thought you didn’t want to play this game.  
Sunburst: Well, I didn’t want to pretend to be a foal, but I know you wanted to play. I thought it’d be fun to play a version big enough for full-grown ponies. What do you think? Fun, right?
 
S08E08 - Sunburst: I don’t even have to go home. She sends me letters constantly asking about my plans for the future. [imitating Stellar Flare] ‘You’re a grown-up pony, Sunburst! You need a plan!’ [sighs] I haven’t been home in a while either.  
Starlight Glimmer: [scoffs] My dad won’t even let me talk to anypony in case they’re ‘strangers’. I’m a grown mare!  
Starlight Glimmer: And you’re not any better! I’m not a filly! I’m a grown pony with a job to do, and I’ll do it on my own!  
tarlight Glimmer: Dad?  
Firelight: Hello, Ms. Glimmer. How is your friendship problem search going? I only ask as an interested observer, since I know you are an adult who’s capable of doing things without help from anypony else.
 
BABScon 2014:  
https://youtu.be/TUktBarg7YQ?t=3829 “
 
Now you just need to do the math.
 
But I’m not talking about EqG, EqG is another universe and the mane6 are teenagers there.  
Also, while Maud has a doctorate in MLP, she is still a high school student in EqG:  
Ishi said that the mane6 from EqG are around 17-years-old.

Rainbow Dash is 21. Just count the candles. ;)
 
The mane 8 are of course 17-18 in EQG. ;)

The mane 6 in Equestria have always been early 20s for me. They would be mid 20s by now.

@Background Pony #500F  

 
They’re always casting twenty-somethings as high-school students. What can ya do?

@Wiimeiser  
Well I went with the characters ages all being one year apart in a row, which is neat if not entirely realistic. I decided their ages at the time of marking to be 8 (Twilight, who is a young adult just entering the world at the beginning of the show), 9 (Rainbow - one of the most immature), 10 (Pinkie - one year younger than…), 11 (Fluttershy - definitely on the older end given her body shape, but not one of the most “grow-up” two), 12 (Applejack, looks small in her flashback but said that all Apples get their marks late) and 13 (Rarity, an established business owner with her own brand by the time of the show)
 
Then, I decided on the gap, which is also Spike’s age. Given that he never goes to school in the show, I would prefer him not to be too young, but 11 years was about as far as I could stretch. So that makes the Manes about 19-24 as of Luna’s return.
 
One reason I could only strtch the gap that far is that it has long been implied (and is now confirmed) that Apple Bloom wwas born before Applejack went to Manhattan. Hence, her minimum age will be about a year ahead of Spike. And her being older than 12 at the start of the show would be a stretch even if she wasn’t still in school several years later. The other CMC are probably a bit younger, since they weren’t in her class yet in Call of the Cutie.
 
Now as for birthdays, that’s trickier. We have see several of them in the show, but no dates were given. At most, we can determine the seasons. (Will fill in later)
 

 
 
@Wiimeiser  
Plaid is a young teenager. Zephyr is, IMO, about three years younger than Fluttershy (and therefore by the above, only one year younger than Rainbow.)

@Ferrotter  
Er, I agree with most of your reasoning (might even switch AJ and Rarity on my list, I’m not sure why you think Fluttershy and Pinkie are older than the Ponyville two. Is there an unspoken assumption in there based on Fluttershy’s physical development? Because some folks are just early bloomers.