Presumably, yes. Most of the campaigns I have played in have not ended in a TPK, and as there were no further adventures in that particular campaign world (or that part of the world), it could be assumed that they had no further adventures and did die of old age. I would guess that such characters would count as retired.
I suppose it's the Schrodinger's Cat of gaming; the only PCs whose deaths I am sure of are the ones I was playing (or playing with) when it happened.