While the Pope certainly CAN leave step down, stepping down under these circumstances is a huge break with the 2000 year old tradition of the Catholic Church and the understanding that the papacy is a lifetime job in all but the most extreme of circumstances. Many Popes is far worse physical shape than Benedict XVI continued to reign.
There is no written rule about the length of a Pope's term in the office. That most of the Popes in history have stayed in the office until their deaths doesn't mean that is the requirement for everyone who serves. The Pope is not the Church, he is simply the servant who happens to be the head of the world's Catholics. Pope Benedict realized that his health was failing and that being the leader would require much more strength than he had, and staying in the office would be detrimental to the work of the Church.