Now we're delving into the really confusing stuff!

So I'm pretty sure that anyone who's been watching Equestria Girls has looked at the timeline its given and has been utterly confused by how it works. From the look of things it doesn't make any sense. Sunset Shimmer was Celestia's Student before Twilight Sparkle and fled through the mirror after she became too ambitious and has been gone for a long time. The odd thing is when you listen to Princess Celestia's story in Equestria Girls, look at Sunset's backstory in the comic and then look at the Human world, things just don't seem to match up, leaving many FiM fans stumped.

Well hold on to your seats folks because I have an explanation for this! You also might want to get an ice pack or bag of Frozen peas as explanations on time/dimensional travel tend to give people massive headaches.

As I've mentioned the timeline in Equestria and the Timeline in Pedestria doesn't match. If you look at Sunset Shimmer's back story in the comics and listen to Celestia's story, Sunset Shimmer was Princess Celestia's student before Twilight Sparkle was. She was being groomed to be an Alicorn princess much like Twilight was as the comics have left plenty of hints.

• In the Fiendship is Magic: King Sombra issue it was explained by Radiant Hope, when she got the letter calling for her to come to Canterlot so she could be a personal student to one the Princesses, that a Princess only takes on a student if they have to potential to be a princess. This was one of the triggers that sent Sombra down the path he took and may be one of the reasons this became a secret 1000 years in the future.
• In the "Fall of Sunset Shimmer" Origin Comic when Sunset Shimmer looked into the mirror and Celestia asked what she saw, Sunset saw herself as an Alicorn Princess (Similar to her Friendship Games appearance) and said in her own words she sees somepony powerful enough to rule Equestria.
• And when Sunset Shimmer demanded that Celestia make a princess, Celestia said in her own words "No, being a Princess must be earned...I have been trying teaching you everything you need to know, but you've turned from it. Every time you say you "deserve" something without the effort just proves to me that you're not ready."
• There's also the possibility that she was being groomed to be a potential Element of Magic as Celestia has also been trying to hint that Sunset should make some friends as she's been trying to tell Sunset how valuable Friendship is.
  

However as we all know Celestia's plans fell through and Sunset fled through the mirror. The same comic also pinpoints the time she left as when you look in the background you will see a filly Twilight Sparkle being babysit by Cadence who was 17-18 at the time according to the Neigh Anything comic where it indicated that Cadence and Shining Armor where High School seniors. And when you look at Sunset Shimmer you can also see that she's the same age as Cadence as she has the appearance of a young adult mare and is still attending the School for Gifted Unicorns.

Now this is where we get into the wibbly wobbly, timey wimey stuff. As many fans have noticed the time frame doesn't match up as in the human world, judging by the photos in Principal Celesta's office at CHS, Sunset has been in the Human World for a total of four years but in Equestria many more have passed.

Let's take a look at Twilight Sparkle for a second. When Sunset left, Twilight was a filly at the Age 5, maybe even 4 as the comic indicates that Sunset left a few days after she passed her midterm, being babysat by a 17-18 year old Cadence. Fast Forward to the events of Friendship is Magic where Twilight Sparkle was sent to Ponyville as a young adult, this would put Twilight around the age of 18-24 years of age however I assume it's somewhere around the minimum. This means that while time passed in four years in the human world, time passed in Equestria at a minimum of 13 years (3 years and 3 months in Equestria per year in Pedestria) or a maximum of 19 years (4 years, 9 months Equestria per Pedestrian Year). At this point it's fair to make the conclusion that time isn't synced between the two worlds.

Now going back to Sunset. As we all know, the Humans are either aged up or aged down in comparison to their Equestrian counterparts and the Mirror also has that effect on those who use it. While Sunset did leave Equestria as an 17-18 year old mare the mirror aged her down to a Human with the physical age of 14 years when she entered the Human World. As of EQG: FG she has now has the physical age of 17-18 Year Old human. But that's not all, the mirror also seems to have the effect of freezing a pony's physical age in Equestria as Sunset has returned to Equestria in Equestria Girls and it looks like she hasn't aged a bit.

With that said, while she may still have the appearance and physical age of a young adult mare officially in Equestria is a total of 30-31 years old minimum, 36-37 years maximum. That's just going by the time that's passed in since she was born in Equestria. Her actual age is 21-22 years old (17-18 years of life in Equestria, 4 years of life in the human world)

In other words, she's going to need some new ID because that's not going to fly!