Thinning of hairs grow with the age. At the end person become bald, if proper care wouldn't be taken. Common causes of hair thinning includes harmonal imbalance, stress, nutritional deficiency, lack of proteins etc.
If some one wants to get it treated then you must be consult a hair loss specialist. start treatment with medications. If no improvement then you have to go under hair transplant surgery which is the only permanent & natural hair loss treatment.