I decided to do this yesterday, because this is obviously more important that my english exam  
Ok, first of all we have their basic heights and body structure, and are arranged in order from oldest to youngest.
 
Applejack:  
When she was a filly, she was always heaps bigger than all the others, and that coupled with her rough exterior and outgoing nature gave her a bit of a self-esteem problem, because she thought that most of the other fillies were scared of her or something (they weren’t, of course, but she thought they were). Because of this, she would always try to dress and act ‘girlie’ because thats what all the other female fillies were doing and she tried to fit in. After a while, she realised that trying to be someone she wasn’t was stupid, and so she started being more herself (this is when her cowboy hat comes in). She didn’t continue school after primary school, because she just wanted to work on the farm, which is why she isn’t shown to be at school in the ‘Cutie Mark Chronicles’, while most of the others are. In this picture, she would be about 10 years old. (This is one headcanon I have for her, but I also have another one curtesy of a picture on tumblr, where she and Big Mac were born with swapped genders and then transition into what they are now ie Aj MtF, and Big Mac FtM.)
 
Fluttershy:  
She was that quiet, hippie kid in class who doesn’t participate in anything and doesn’t have any friends, and refuses to wash and cut her hair. She did have Rainbow Dash, but they weren’t really ‘friends’ per say, as they were that Rainbow Dash was just her protector from bullies. They didn’t hang out, but Dash was nice enough to her, and always stood up for anyone if they were getting bullied. After the race that got them their cutie marks, they actually went their separate ways for quite a bit before they both moved to Ponyville and got back in contact again. I don’t actually think they were that close friends until after they finished flight school, because Dash, although she liked to look after everyone and enforce ‘justice’, she still had quite and obsession with her rep, and so didn’t want to be caught hanging out with someone who was considered a ‘loser’ too often. Fluttershy also loved decorating her mane with flowers, as shown here, but would never go to school or show anyone else because she was afraid they would mock her, but she always dreamed of revealing her secret to someone and receiving compliments and maybe even get a friend out of it. She fills out, and gets very tall, but no where near as skinny and scrawny, and is actually considered quite beautiful. She is born in the same year as Rainbow Dash and Rarity, and would be about 9 here (she was born earlier in the year than the others). She went to flight school with Dash, which is the Cloudsdale equivalent of normal school, but with a higher focus on flying, and at this time Rarity was also at school.
 
Rainbow Dash:  
As stated above, she had a thing for her rep, and was overly protective of it. She was also a lot more arrogant and would pick fights left and right. She has (and still does to an extent) a high sense of justice, and a burning desire to do what’s right, though her head strong nature can cause this perception to get skewed. Her tiny statue has also fuelled her hot-headedness, and she doesn’t really grow that much, but gets skinny and lanky, much like Fluttershy was as a filly. Her over-confidence was used to hide her fear of failure, something she doesn’t get much better at hiding. She won numerous awards while at the school, and was one of the top students, athletically. She was born in the same year as Fluttershy and Rarity, and would be about 8 here.
 
Rarity:  
Rarity was a lot more ‘snobby’ as a filly, as she thought her ‘refined’ behaviour was above those around her. This, much like Rainbow Dash’s arrogance, eventually smoothed itself out, though has left remnants in her personality today. She was still very generous and kind, but her confidence sometimes made her come across the wrong way. She was born in the same year as Fluttershy and Rainbow Dash, and would be about 8 here.
 
Pinkie Pie:  
Absolutely crazy, as obvious from the show and the picture above, she never went to school because she was expected to work on the rock farm. When she moved to Ponyville, the Cake’s gave her basic home schooling, enough for her to read and write well, and do basic math. She is born in the same year as Twilight, but is older than her, and would be about 7 here.
 
Twilight Sparkle:  
She entered Celestia’s school as quite young, much younger than normal, and her natural affinity to magic was impressive enough to land her a spot as the Princess’ personal apprentice. She was always shy and had no friends, partly because of the slight aloof vibe she gave off to new ponies she met. She was born in the same year as Pinkie, and would be about 6 here.
 
I’m also gonna do a thing like this with their adult/current forms, and anything else I can be bothered to do. I might also add to this too.  
Hope you like it!  
Art © Me  
Characters © Hasbro