I'm looking right now at the Wiki page on papal resignations (yes, famous last words I know) and prior to Gregory XII's resignation in 1415 it says Celestine V resigned in 1294. Before that the circumstances of earlier resignations seem to be shrouded in a lot of historical fog. To be more specific, a number of these earlier resignations happened under some element of intrigue or even outright bribery -- particularly the resignation of Benedict IX in 1045.
This earlier Benedict resigned the papacy at age 33 in order to get married. He also did so to collect a bribe from the guy who replaced him. So this seems like a categorically different sort of affair and not really comparable it to his namesake's resignation today.
From what I can tell the really apt comparison is to Celestine V in 1294. Even through the fog of history, Celestine resigned apparently not under coercion or for some dishonorable purpose but because of declining physical strength and the desire to go back to being a monk. Unfortunately for him, his successor Boniface VIII had him placed under house arrest where he died less than two years later.
I know we have historian-readers with relevant knowledge about this question. Please chime in.