Is the oldest ring on a tree the middle ring or the outer ring?

The inner ring. The tree grows a pith up the middle and grows outward from there. The summer wood, which grows in the spring and summer, makes the bulk of the wood, and the winter wood, grows in fall and winter, makes the darker, denser wood, referred to as the rings. It is because of the switching between the summer and winter wood that you can usually determine the age of a cut tree to within a year or two.