I mentioned this in a blog post, but no one really ran with it. Think about it, the movie said that the Sirens were banished from Equestria when Starswirl was alive; and as we know, that was a very long time ago. Now, true, the timelines on Earth and in Happy Horse Land aren't exactly the same -- proof; as seen in Pinkie Pride, Rainbow Dash is twenty-one, but as a human, she can't be older than eighteen, because she's in high school.

So what we know is that Starswirl banished the Sirens a long time ago (probably a few centuries), and that Earth and Equestria don't match up. Either way, wouldn't that make the Dazzlings too old for high school? Unless they were sent through the mirror when they were fillies (or whatever baby sirens are called)?

...I am confuse.

Well,Twilight at the end of the movie said:''without those pendants and the magic you brought here from Equestria,they're three harmless teenage girls!''.
So,I suppose They're teenagers.

3682397 I don't think they age.

3682397 Idunno if this strictly complies to the official materials, but I have this convoluted headcanon that time between Equestria and the human world only matches up properly when the portal is open, and the longer it goes without anyone passing through and thus refreshing the connection, the more Equestria's timeframe 'speeds up,' so to speak, relative to the human world's progression. Thus the Dazzlings were sent through some thousand+ years ago and have only been in the human world for about eight, and yet Sunset went through about ~eight years ago in Equestria but spends three in the human world.

I'm of the headcanon that Starswirl sent them a thousand or so (however long ago it was that all that exciting backstory for all those villains apparently happened in a short span of time, like Starswirl, Luna, and Celestia were the only heroic figures with their own regular climactic battles to fight up until Twilight popped up or something) years into the future, so it's only been months for them.

That or Aria has only gotten tired of 'fast food' after about a millenium of it, as it sounded like Adagio was explaining for the first time that the energy wasn't the same as in Equestria. Or maybe these three have Alzheimer's and just haven't physically (or mentally, perhaps, going by Sonata's childish sort of demeanor) aged in all the time they've been there, but forget the countless times that exact conversation must have come up by now.

If it's the first one, they'd be about as old as they were when they were banished, I guess? If they showed up in the other world as teenagers, I would guess they were adolescents (for their species) in Equestria as well, but I get the impression Sunset and Twilight are roughly adult ponies and the same thing happened to them. Who knows?