This hadeeth has been narrated in al-Saheehayn in several versions, such as that narrated by al-Bukhaari (6146) and Muslim (2952) from ‘Aa’ishah (may Allaah be pleased with her) who said: “Some tough men among the Bedouin came to the Prophet (peace and blessings of Allaah be upon him) and asked him when the Hour would be. He would look at the youngest of them and say: ‘If this one lives, old age will not catch up with him until your Hour begins.’” Hishaam [one of the narrators of the hadeeth] said: This means their death.
The hadeeth is quite clear: what is meant is that the Hour of these people, i.e., their death, was something that was close at hand, and would happen when this youngster grew old. This hadeeth does not refer to the greater Hour which is the Day of Resurrection.
Al-Qaadi said: What was meant by “your Hour” was their death; it meant that that generation would die, or that those who were being addressed would die.
Quoted from Sharh Muslim by al-Nawawi.
Al-Karmaani said: This answer was very wise, i.e., it was a wise way of telling them not to ask about the time of the greater Hour, for no one knows that except Allaah; rather they should ask about the time when their generation would come to an end; it is more appropriate to know this because it will motivate you to keep doing righteous deeds before it is too late, because none of you knows who will die before the other.
Al-Raaghib al-Isfahaani said: The word saa’ah (hour) means a portion of time and is used to refer to the Resurrection, to indicate that the Reckoning will be very swift. Allaah says (interpretation of the meaning):
“and He is the Swiftest in taking account”
Or it may indicate the meaning stated in the following verse (interpretation of the meaning):
“On the Day when they will see that (torment) with which they are promised (i.e. threatened, it will be) as if they had not stayed more than an hour in a single day”
The word saa’ah is used to refer to three things: al-saa’ah al-kubra (the greater Hour), which is when the people will be resurrected to face the Reckoning; al-saa’ah al-wusta (the middle Hour) which is the death of a generation; and al-saa’ah al-sughra (the lesser hour) which is a person’s death. So the Hour of each person is his death.
From Fath al-Baari.
And Allaah knows best.