I think there's a big difference between 1) exploring what characters might have been like when they were younger, how they got to be the way they are today, and whether there could be any surprises in their past, and 2) de-ageing characters but maintaining the exact same personalities, mannerisms, tics, and often, inexplicably, similar levels of experience and knowledge as their older counterparts. The latter is also awkward if part of a character's backstory is something that happened when they were older, like the death of a child or military experience, but you're writing them at a point in their life where that hasn't happened yet.