During the live letter, Yoshi-P said he's hesitant to have the twins age up because he's worried about the faction of players that will go "YOUNG ALPHINAUD/ALISAIE WERE BETTER" and if they were to age them up normally, there's no going back.
So here's what I propose as a best of both options: a non-MSQ side story quest series, that I like to call Visions of Possible Futures.
Have our WoL, curious to learn more about our Echo abilities, meetup with Mikoto from the Ivalice/Bozja quests once again. With our ability to see and re-live the past, and her ability to see the future, have our dynamis abilities react to the two blessings and trigger a series of flash forwards, where we experience and live out a vision of what our world might be like in one of many possible years-later futures, where everyone is noticeably older. This would be the perfect excuse for literally limitless What If scenarios for how not only the twins, but the other Scions, and any other characters, might be living years from now, and because it's us projecting ourselves into the future, we can get away with not aging our own character up as well, so no changes to the player need be made, though at the same time, players would be free to re-customize themselves to look older when playing these quests if they so choose. Maybe to that end the quest could offer a free Fantasia or two? It would be a perfectly acceptable way to experiment with aging up story characters in a non-committal way. And because it's just "one of many possible futures," everyone is 100% free to accept these stories as canon or non-canon as they please, and the MSQ can proceed with the current timeline as-is with no irreversible changes.