What is the cause of mouth sores?does it get many at age 30yo above? how and what to do to remove it?

Depends: Mouth sores could cover many different things. You should see your dentist find out just what is going on and then talk about treatment.

Mouth sores: Many possible causes such as smoking, dipping, bacterial/viral/fungal infection, systemic disorder, trauma, neoplasm, and so many other possibilities. See your General Dentist for differential diagnosis and then appropriate treatment, referral, or observation.