The FDA has approved Zostavax for patients 50 and older (http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm248390.htm).
Prior to this the vaccine was available for patients 60 and older.
Read the following link which summarizes the study that helped in this decision: http://www.internalmedicinenews.com/index.php?id=495&cHash=071010&tx_ttnews[tt_news]=53263