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MLP seems to be in the transition from pre-industrial to industrial society and Canterlot is the equivalent to London so I would equate Celestia's school with Eton. Entrance exam is at 7 and Twilight only mentions Kindergarten prior. . Being the overachiever that she is and Celestia's student, I see no reason she has not already completed her requirements for several degrees and was working on an independent study project or 4 at the shows opener.
Rainbow Dash had 21 candles on her cake. She and Fluttershy went to flight camp the same year but I suspect Fluttershy is a few months older but they seemed to know each other prior to camp so I put Fluttershy also at 21, making Pinky 20. Leaving us Rarity and Applejack. Applejack states she was younger than Apple Bloom but the oldest in her class. Placing her in Rarity in different classes/ages if we assume Rarity was already in Ponyville at this point. That places her a year or two behind Applejack since Ponyville seems to have only a one room schoolhouse. Apple Bloom is the only one left in her class without her Mark until she meets Sweetie Belle and Scootaloo. I think Sweetie Belle and Scootaloo are a year behind Apple Bloom as they don't seem to stress quite so much about being late getting Cutie Marks. In 'Some Pony to Watch over me the Apples decided Apple Bloom was old enough to stay home by herself during the day. So I put her at 12. For there early industrial society this would be the age a pony would begin an apprenticeship as seen in the recent Apple Bloom Lunable episode she expected to start her apprenticeship as soon as she got her Mark. When Applejack leaves for Manehattan we only see Granny Smith and Big Mac. I'm divided on my theory here. Apple Bloom could have been an infant down for a nap inside and their parents already gone, but given the huge role she plays in AJ's life now and was the audience I think Applejack would have mentioned leaving her behind. Listening to my grandma's stories of the family farm, all the children from the extented family would be left at the main house to be watched by the grandparents during the week to go to school while able bodied adults worked long hours in the fields often going to trade labor at other family farms in nearby towns. So I don't think that Apple Bloom was born yet but the news of a new sibling may have prompted her to want to move to a different relative's care. Unless Equestrian tradition has only the Alpha Mare inheritance laws, which is entirely possible I think Big Mac is younger than AJ either by a year or 2 or they're twins. So if Applejack is slightly less than twice the age of Apple Bloom, she is 23 making Rarity 21-22. Of note Apple Bloom seems to be one of the older kids in the schoolhouse and mandatory schooling for that type of school/Era would be 14. Alternately Wiki places Apple Bloom's VA at 7 at the start of the show. This more closely matches with my estimate for Twilight lending credit to the theory Apple Bloom was born after the sonic rainboom. In fact I like the theory the Apple siblings are siblings because Granny said so and are in fact cousins ophaned in the extended clan. The former theory places Spike at 13 and Making Twilight Sparkle 20. Side note Spike is no doubt home schooled and as Twilight seems to confuse him with a pet, top of his class in obedience training. Even the writers seem divided on Spike's age as a dragon. In 'Castle Sweet Castle' he graduated from a crib in Twilight's room to his own room with a 'big boy' bed and his blanket, plush Rarity and the toys we see in it are very typical of my toddler nephew. While he is also a comic enthusiast with good preservation habits, Is expected to keep up with the castle's house work and stay overnight by himself and care for several pets, and talks football with Big Mac. When he is away from Twilight he seems a typical teen, only reverting to a 'baby' dragon when reminded.