What would cause eye color to change and dull at the age of 23?

Examination: You need to go see an physician. I can't think of good things that make your eyes change color and become dull.

Sunlight: Although it is not typical some people have light sensitive freckles on their iris that can darken with cumulative light exposure. Since eye color is a composite effect this is one possibility. Another is a condition called Fuchs Cyclitis which often makes one eye duller or heterochromic to the other. Both possibilities should be investigated by an Eye MD.

Iris pigment : normally remains stable early in life. If at your young age you notice an attenuated color of the of the iris, see your Ophthalmologist . They will look for issues of corneal clarity (possibly a corneal dystrophy causing edema) or iris pathology as well. All manageable if diagnosed early.