John Calvin Commentary Numbers 26:63

"These are they that were numbered by Moses and Eleazar the priest, who numbered the children of Israel in the plains of Moab by the Jordan at Jericho." — Numbers 26:63 (ASV)

These are they that were numbered. The former registration was made by Aaron. The difference between the two census is therefore specified, so that Moses might take the occasion to commemorate God’s judgment (which can never be sufficiently considered), that out of 600,000, only two were found who had survived to the term prescribed by God.

If anyone should object that the greater part would have died naturally, since they had arrived at their thirtieth, fortieth, and even fiftieth year,¹⁹⁸ and thus would have been about eighty years old before the completion of the forty years, I admit that this is the case. However, many had not yet reached their twenty-fourth year. Nor can we doubt that not a few of them were younger than Caleb and Joshua, whom we know to have been not only alive and well but also strong and vigorous for many years afterwards.

At any rate, therefore, not old age, but God’s vengeance, cut off half of them by an untimely death, as if He had openly put forth His hand from heaven and struck them. It is not without reason, then, that Moses states that they were dead, as God had pronounced; not merely that by the punishment inflicted upon them He may instill in us the fear of God, but also that we may learn to be genuinely stirred by His threats.

¹⁹⁸ This sentence is omitted in the edit. of Geneva, 1563.