One possible magic ingredient to age a character's voice could be a potion made from Elder Tree bark. The Elder Tree is known for its associations with wisdom and age, and by infusing its bark into a special brew, it could have the magical ability to add a nuance of maturity and depth to a character's voice. This potion could be consumed or applied directly to the vocal cords for a temporary effect, allowing the character to sound older and more experienced, even if they are actually quite young.