It's very unfortunate that you are having sensitivity in molars at such a young age.
Sensitivity could be due to multiple factors, like, cavities or erosion of enamel or notches in the teeth, if you are using a very hard brush or if you have a horizontal brushing habit.
Also sensitivity could be due to severe grinding habit, which may lead to attrition of enamel.
Advise you to visit your local Dentist and solve the problem.
Hope this clarifies your doubts. In case of any other query, please feel free to get back.