My headcannon is  
At show start  
RD 19 celebrate 20 with her parent (my mom like doing something cool for decades. I turned 10 at Disneyland. At 20 my sis got an electric guitar. In advence for our 30 (and 40 probably) we got our own condos morgage free. Now her parent they celebrate her lowest achievements theres no way they would miss on that) then turned 21 in pinkie pride then wonderbolt academy happened since she had the required age to enter the military during peace time.  
Pinkie 22  
Fluttershy is 24 (older then Pinkie by 13 to 17 months)  
AJ & Rarity are around 23 (13 being a sensible age for AJ to be trusted to travel alone on a train, and Rarity’s parents adopting a more laisser-faire attitude toward parenting.) Rarity being the eldest of the two.  
Twi she is the 16 genius.

@FanOfMostEverything  
Actually, unless there’s a Summer Sun Celebration offscreen, you’re about right. S1-S3 takes place over the course of one year in-universe.

These are some very nice humanizations. My only issue is the implication that all four seasons have occurred over less than a year. (And yes, I know there’s an argument for it, but I’d really rather not get into that.)

Fluttershy looks embarrassed to be tall. Seems about right.