I am not from the US, which means that my concept of American/Canadian schooling systems is limited;
However, using Australian schooling systems, I have worked out somehow what age each member of the human Mane 6 and Sunset Shimmer is.
For starters, we should look at the obvious things, Freshman Year Down Under is years 7 and 8. We can assume, because high school starts when you're 12, Sunset arrived in the human world and became the age of 12. In reality, Sunset is actually 4 years older than the actual age she is; when she's 12 or 13, she's actually 16 or 17, and when she's 16 or 17, she's 20 or 21. The way I got this is because Twilight, in Pony years would be 18, meaning that the portal ages you down or up depending on where you have to be. I also got this because of the fact that Sunset has won princess of the fall formal 3 times prior to the 1st EQG movie.
Using info from the show prior, we know that Fluttershy is a year older than Pinkie. Therefore, Fluttershy could also be a year older than other members of the cast, being 16 during the first 2 movies, and 17 during the third. She could have been dropped down a grade, or started kindergarten at the age of 6.
Using this info, during the first movie, Pinkie and Rainbow are possibly 15 during the first two movies. Though I think, due to the episode Pinkie Pride happening inbetween EQG 1 and EQG 2, Rainbow was 15 during movie 1, and turned 16 before movie 2. She's also 16 during movie 3. Pinkie however, is 15 during movies 1 and 2, and 16 during movie 3.
Applejack and Rarity are the hardest, mostly because of their younger siblings. We can only assume that there's a substantial age gap between the pair, as well as AJ and Big Mac. Most likely, if he's of similar age to Shining Armor, he was probably 16 when AJ started and 18 or 19 during EQG one. Therefore, AJ was 13 when she started, 16 during EQG 1 and 2 and 17 during EQG 3.
Rarity though would possibly be similar in age to Twilight and Sunset (as she appears). We could assume that there's a three year age gap between her and Sweetie Belle, meaning that if Sweetie's 12 during EQG 1, she's 15.
Finally there's Twilight, we'll use a line from Principal Cinch, where she mentions that Twilight is on her last year at Crystal Prep. This means that Twilight could be in Year 11 or 12 and either be 17 or 18.
I'll assume because Rainbow is allowed to run a pep rally for CHS that the Humane 6 plus Human Twilight are in year 11, because where I'm from, the Year 11's are considered to be the Leadership Class alongside the Year 12's.
But that's my opinion. And it's a lot of writing. So TLDR the ages in EQG 3 are;
Twilight: 18
Sunset: 18 or 19 (Pony ages 21 or 22)
Pinkie: 16
Rainbow: 16
Fluttershy: 17
Rarity: 16
Applejack: 17