She glares into her counterpart's eyes and tells her that it's over. She then lets her crumple to the ground and watches as she dies in Princess Twilight's arms.

Why did Sunset Shimmer turns evil?

However, Sunset steals the crown from Twilight, puts it on, and transforms into a demonic-looking winged humanoid due to her wickedness in her heart. In this form, Sunset becomes purely evil. With her new powers, she plans to use them and her army of mind-controlled teenagers to conquer Equestria.

How old is Sunset Shimmer now?

If we assume the mane 6 are young adults, 20ish, in the series, then Sunset would be late 20 to 30. A bit old for high school, but appearances behind the mirror may not reflect their proper age.