Dandruff at the age of 22 is due to hormones which increases secretion of oily substance to the skin. The scalp gets sticky and absorbs dirt. That too from a city like Chennai It's too bad.
A natural anti dandruff shampoo without chemicals is good. Rinse your hair once a week. Don't do daily as it will weaken you hair strands. If that get rids of the issue then good.
If it's too itchy the reason should be fungal growth on scalp. For this you have to take medication. There are quite a lot of safe treatments in Siddha system.