Here's a breakdown:
* She's a witch: Juliet is a witch, and witches in the Wizards universe have a longer lifespan than humans.
* Magical beings age differently: The show doesn't explicitly state how long witches live, but it's implied that their aging process is slowed down by magic.
* The show's continuity: The show focuses on the three Russo siblings, and their ages are consistent throughout the series. Juliet's unchanging appearance reflects the show's focus on the family dynamic and their magical adventures.
It's important to note that this is just a fictional aspect of the show, and not intended to be realistic.