I actually have a theory about that, when StarSwirl the Bearded made that mirror portal, he customized it to not only work at a specific point of time, but made it so that it would transform who ever used it into a form that would allow them to blend in with the inhabitants of that world, but an unintended side-effect of that is that once you enter that mirror & take on that new form, you also STOP AGING, and only by entering the portal to return home do you resume aging at your normal rate, which would also explain how the Sirens from the 2nd EQG have managed to live since the time of StarSwirl the Bearded to today, I mean even if they were immortal their natural Equestria Magic was neutralized upon entering the human world & taking on human forms, and even if their amulets were a source of power for them, the properties of Human world didn't work the same as Equestria, and when the amulets were destroyed the Sirens didn't age to dust, so the Amulets couldn't have been a source of eternal youth for them, and they couldn't generate enough magic to perform any Age Spells to keep themselves young & vital or anything, so the best explanation would be that while in Human Form they stopped aging, which would also mean that while Sunset Shimmer was in Human World & in Human Form she stopped aging as well, which means she's been stuck looking like a teenager for almost 10 years or something, which would have given her plenty of time to learn how to adapt to the Human World & figure out how to become a big shot there, plus if there really is a second version of every pony from Equestria living in the Human World, then that would indicate that Sunset's Human Counterpart(the Real Human Sunset) is somewhere in her mid-20's, which would possibly be why we haven't seen her anywhere in the EQG franchise yet, and just imagine how it would turn out if Pony Sunset decided to go back to living in Equestria so she can make amends with Celestia, be reunited with her real family again & start a new journey to make as many new friends as she possibly can throughout Equestria & beyond, and then the next day in Human World, the Real Human Sunset would come to Canterlot High as the Human Mane 6's new Homeroom Teacher, now THAT would be something I'd like to see happen, anyways I hope that this will help answer your question.