* My Little Pony: Friendship is Magic doesn't use conventional age: The ponies don't age in the same way humans do. While they mature and develop, their physical appearance remains relatively consistent.
* Twilight's age is ambiguous: The show never explicitly states Twilight's age. We know she's a young pony, but it's unclear whether she's a teenager, young adult, or somewhere in between.
* "Magical Mystery Cure" marks a major turning point: The episode sees Twilight becoming an alicorn, signifying a significant developmental leap. However, it doesn't clarify if this instantly makes her "older" in a conventional sense.
Based on the show's overall tone, we can infer that Twilight Sparkle in season 3 episode 13 is somewhere between 16 and 20 years old. This aligns with her character arc and the themes of growth and responsibility present in the show.
Ultimately, the exact age is left open to interpretation. The focus is on Twilight's internal development and journey rather than her precise chronological age.