Dis anyone in your family go grey prematurely? My brother-in-law went totally grey at 17, just like his mother did. If it's an inherited trait, there's little you can do.
Make a tea of nettle-leaves (pick them before they blossomed or buy them) in the evening, pour it in a mug and cover the mug. Leave it overnight, drink the tea in the morning before you have breakfast. Do so every day. After several days your hair will have darkened a bit. This is said to be because of the positive effect of nettle-tea on the kidneys, who seem to be co-responsible for the whitening of our hair.