I think the words he chose are very significant. He refers to his conscience, and to "strength of mind".
Dementia, or another age-related illness would be adequately, and arguably less embarrassingly, covered by "strength of...body".
There would also be no need for any reference to his consicence. It would not be a matter for one's conscience as a Pope, I think, to resign due to ill health. He could quite justifiably refer to this being, for example, a sensible decision in the best interests of the Church.
I think this is a considered, moral and political decision. He no longer has the mental energy to deal with everything a Pope, and/or specidically he must deal with in the 21st century.
Whether this is because of his personal history or the Church's history we will probably never know.