Archor

They're definetely potrayed as seniors, so 17-18 years old. They're most definetely the sake age as Dj Pon 3, who can drive, and also has her own car, and Rainbow Dash owns and drives a motorcycle. They're highschool is potrayed like a US highschool, so basing it on that they're not 16, because you can only get a motorcycle license at the age of 16, and since Rainbow owns a motorcycle I think she's at least had her liecense for a year. In the first movie it was also shown that there were 3 fall formals, and this one being sunests 4th, and we know the main 6 are all the same age, so judging from that we know they're seniors. A lot of the main six have also been shown to have jobs, meaning as I already said, they're older than 16, as most places hire at 16+. And as I already mentioned, Rainbow Dash has her own motorcycle. As well as this, it's clear they're all in the sane grade because of the yearbook photo from the first movie(in the photo based on their looks, comparing them to to the cmc they were clearly freshman in that photo) Also as we know from Driving Ms. Shimmer, Fluttershy owns a car, which means she has a license since she offered to help Sunset learn how to drive, which also implies she's older than 16. So yeah, based on all this they're either 17 or 18.