Premature greying of hair (in 20's ) is a very common complaint these days. And it is alarming, not because it is a cosmetic problem but it indicates body health issues. Unhealthy lifestyle (bad sleeping habits and eating habits and no exercise) leads to excessive oxidative stresses in the body and because healthy habits aren't being practised, these damaging oxidative stresses aren't counter acted. Thus, premature aging of the body (greying is a small part) starts. Please understand that although tablets are there, they are not the only treatment , you will have to approach your problem holistically ... which means start modifying lifestyle . (Takes time but start somewhere ). Your skin and hair will be healthy only if your body is healthy. Do read this article to get a gist of what I am talking about : http://www.skinmatra.com/single-post/2016/09/17/Listen-to-your-skin-talk-Here-is-how

Premature hair graying is usually hereditary.  In addition factors like excess exposure to sunlight or use of hair colors or treatments and sometimes stress or nutritional imbalance can lead to early graying. Medically food supplement with biotin and calcium pantothenic acid are available to arrest graying. But converting back of gray hair to black doe not happen.