Does a person know of the approach of their death, and is it permissible to supplicate for a long life?

Some people may sense the approach of their demise if there are signs of it, but no one can know precisely when their end will be. This is part of the knowledge of the unseen (Ghaib), which Allah has exclusively reserved for Himself, as confirmed by His statement: "Indeed, Allah [alone] has knowledge of the Hour and sends down the rain and knows what is in the wombs. And no soul perceives what it will earn tomorrow, and no soul perceives in what land it will die. Indeed, Allah is Knowing and Acquainted." As for supplicating for a long life, it is permissible under certain conditions.