Some times hair whitening in some people are normal. Various supplements are claimed to be useful for it.
vitamin B and antioxidant deficiency, smoking, alcohol,
thyroid hormone excess or deficiency etc are thought to be factors that may influence.
Most times premature graying of hair has no identifiable cause. But you now know the supposed factors.
Don't use a shampoo everyday. Any of the 'good' commercially available shampoos are ok, if you do not have any particular allergies or issues with it
Next Steps
Wouldn't suggest specific treatments, but B-complex capsules for a couple of months, cutting down on smoking or alcohol, getting a blood test for
thyroid hormone excess or deficiency are all things that usually wouldn't harm and usually worth a shot. Remember to eat more veggies. There are also capsules with antioxidants like vit. E etc.
A consultation with dermatologist can give you some ideas or advice. Also it may be a bit safer for you to ask which supplements to try.