What causes tooth sensitivity, and when can it be reversed?

So many possibilities including carious lesions, recession, bruxism, recession. You should really see a local dentist for an evaluation.

AT YOUR AGE YOUR SENSITIVITY COULD BE FROM USING A HARD TOOTH BRUSH THAT IS WEARING AWAY YOUR TOOTH ENAMEL OR GRINDING YOUR TEETH AT NIGHT.A CAVITY WOULD CAUSE YOU PAIN,IF YOUR GUMS ARE RECEDING FROM POOR ORAL HYGIENE THAN THAT COULD BE A CAUSE. GO FOR CHECKUP TO YOUR DENTIST.