The age-regression spell is a spell used in Vonderland to turn oneself into a baby or others into a baby. It is cast by waving one's wand in a circular motion above their head. The end result is the caster is turned into a baby and conveniently wearing a nappy.
The spell's effect can be lessened so it only takes off a certain amount of age (e.g. when Verne tries, it he only becomes 5 years old again).