If we go by the comics, Fluttershy is the oldest. If we don't, it's still safe to assume that she's the oldest. She was taller than the others. So either she had an early growth spurt, or she was the tallest. Rarity and Applejack were late getting their cutie marks, so I think that means they are on the older side. Rainbow Dash was the first in her class to get her cutie mark, so that means she was younger. I'd say she was the youngest. Pinkie is on the younger side, and she's a year younger than Fluttershy. So Fluttershy, Applejack, and Rarity are around the same age. I think Twilight and Pinkie Pie are the same age as well, and Rainbow is a year younger than them.
In Pinkie Pride, the cake said Rainbow is 21. Assuming that's true, their ages (As of Pinkie Pride) would be as followed:
Fluttershy: 23
Applejack: 23
Rarity: 23
Pinkie Pie: 22
Twilight Sparkle: 22
Rainbow Dash: 21
The maturity of the pony does not determine their age. I was going based on the show for the most part. Applejack and Rarity were both late getting their cutie marks, so that would put them at around the same age as Fluttershy with only months apart. Twilight and Pinkie Pie may have gotten theirs at the right time since there was no indication that they were late or early (that I can remember) so I think they are both around the same age. Rainbow was early, so that would put her as being the youngest. Fluttershy never said she was late, but I think she's the oldest. She is the oldest in the comics, at least. So Fluttershy was also late getting her cutie mark.
Remember, this is assuming that the candles represented Rainbow's age. This is a headcanon.
How much time has passed?
From season 1 to season 4, it has been precisely a year. From Season 4 to now, it's more complicated. If the show is going in chronological order, then it has been at least a year since winter came twice. Maybe from season 4 to the end of season 5, it would have been another year and a half? So if it's Chronological order, I think there have been 2.5 years. Since I doubt it's in chronological order, it's hard to tell. It has been at least one year, that's for sure.