‘And Aaron the priest went up into mount Hor at the commandment of Yahweh, and died there, in the fortieth year after the children of Israel were come out of the land of Egypt, in the fifth month, on the first day of the month. And Aaron was a hundred and twenty and three years old when he died in mount Hor.'
It was on Mount Hor that Aaron was to die ‘at the commandment of Yahweh', the Lord of life and death. This was almost exactly forty years since they had left Egypt, and was on the first day of the fifth month (they had left Egypt on the fifteenth day of the first month). He died at the age of one hundred and twenty three. At first sight this appears to be an exact number, but it may simply be one hundred and twenty (three generations - a man of this age in fact often called himself ‘three') plus three for completeness. Being older than Moses who would shortly die at ‘one hundred and twenty' (Deuteronomy 34:7) the three indicated that he was the elder brother. Moses had had three periods of life, life in Egypt, life among the Midianites, and the period of deliverance, each of which could be seen as ‘a generation' (idealistically forty years).
The mention of his long life here and not earlier was because earlier his sins were still in mind. Now that that has been dealt with Aaron could be given his final accolade. To the ancients his length of life would be seen as evidence of his righteousness. It was evidence that he had been pleasing to Yahweh.