He and the Church are claiming its because of his age, but I bet its political: in point of fact, Ratzinger is currently being investigated for having covered for pedophile priests over the last several decades.
"Citing age and infirmity as his reason for leaving the papacy, Benedict's action comes just weeks after he opened his celebrated Twitter account -- and less than a month after the decades-old child abuse scandal drew nearer to the pope's door, with revelations published in the Los Angeles Times earlier this month that Cardinal Roger Mahony, then Archbishop of Los Angeles, sought to evade the law in cases involving the sexual abuse of children by the priests in his charge by sending them to treatment facilities in states that did not require health professionals to report the crimes to authorities.
At the time that Mahony was covering up the crimes of his priests, Benedict, then known as Cardinal Joseph Ratzinger, led the Congregation for the Doctrine of the Faith, the Vatican office that oversaw such matters.
In archdiocese documents released under a court order earlier this month, Mahony is revealed to have taken actions deliberately contrived to avoid legal prosecution of priests who had sexually abused, and even raped, children. The documents were so damaging that Mahony, now retired and once thought to be a contender for the papacy, was publicly rebuked by the current Archbishop of Los Angeles Jose Gomez, and stripped of any public duties, an unprecedented censure of a cardinal archbishop by his successor.
Amid the cache of church records, released as part of a settlement between the archdiocese and 500 sex-abuse victims, are several letters to Ratzinger from Mahoney, in which the California prelate reports to the Vatican his reasons for various actions (such as defrocking) taken against the offending priests. The records amount to some 30,000 pages, so their full contents have yet to be pored through by investigators and journalists."
Vatican City only exists because the Holy See bowed to Mussolini, and both Italian & German fascism. Catholicism itself is merely an echo of the homoerotic Roman Empire. The Church is both medieval and utterly corrupt.