It's not very important.
Vivio is about 12 years old, through the use of high power enhancement magic, genetic enhancement and being the clone of Sankt Kaiser Olivie basically Nanoha-verse King Arthur/Jesus, she can transform into an 18-20 year old form, which has some advantages in melee combat due to greater reach and leverage.
It's a fairly uncommon magic technique, since the only people who use it much are:
-Old Belka Inheritors - Vivio, Einhart, etc, because the Belkans were big on upgrading their babies to be able to fight if needed and it's heritable.
-Rio - Rio somehow reached it independently by taking the approach of stacking enough strength buffs onto her body to skip to a 16-18 year old form temporarily so she has ROOM to fit in more buffs.
It's pretty much just for combat use, possibly training if she wants to practice in a form with more reach.
It is theoretically possible that adult form would allow her to be take more seriously, but she doesn't need it due to:
-Vivio being known to the Church as basically the second coming of Jesus
-Vivio being a certified librarian at the Infinity Library(which is itself a transdimensional dungeon-thing)
-Vivio's adoptive parents being two of the current three aces. Also her adoptive uncle commands an interdimensional battleship, and a friend of the family is the third ace capable of spells equivalent to nuclear bombardments.
Overall adult form is just for fighting.
She has a weird social group.
