John Calvin Commentary Numbers 33:39

"And Aaron was a hundred and twenty and three years old when he died in mount Hor." — Numbers 33:39 (ASV)

And Aaron was one hundred and twenty-three years old. It is not without reason that the great age of Aaron is expressly stated, since his life had been prolonged to an unusual period for the good of the people. At the age of one hundred, he had already exceeded the ordinary extent of life; whereas, by God’s extraordinary blessing, he survived until the people were about to pass into the promised land. Hence, their ingratitude was the more base in not acknowledging this paternal care of God, since it was for their advantage that He preserved so long the minister of His grace.