I am not aware of any studies that show green tea to have any effect on hair. However, green tea does have beneficial health effects. It is a powerful antioxidant, which means it fights free radicals that harm cells. It also is thought to speed metabolism a bit. As we age, our metabolism slows down, which is partly responsible for the typical weight gain that accompanies getting older. Green tea helps fight this by tending to speed metabolism so that it is closer to the level of a younger person.
Green tea has loads of caffeine, so if you are going to use it, you might consider taking decaf green tea extract supplements.