Antonio Socci in his book shows evidence that Pope Benedict XVI might not have been able to "resign solely for reasons of age."
Benedict's resignation said:
"[B]oth strength of mind and body... has deteriorated... I have had to
recognize my incapacity... For this reason I declare that I renounce the
ministry of Bishop of Rome, successor of Saint Peter."
"An authoritative canonist, Cardinal Vincent Faggioli" under John Paul II wrote:
"[I]n an exhaustive and absolute way, the pope may not resign solely for reasons of age."
(The Secret of Benedict XVI, Pages 67-76)