Once at the Astronomy Tower, Dumbledore and an invisibility-cloaked Harry were confronted by Draco Malfoy, and before Harry could reveal himself, Dumbledore immobilized him with a silent Full Body-Bind Curse. Malfoy disarmed Dumbledore, and admitted that he, Draco, was behind the series of attacks on Hogwarts students, and that Voldemort had ordered him to kill Dumbledore. Dumbledore revealed that he knew of Draco's mission and of the attempts on his life with the Opal necklace and poisoned mead, but allowed him to remain in Hogwarts for knowing Voldemort would kill him if he was exposed and expelled. Dumbledore offered Draco protection, and promised to hide Draco and his mother where Voldemort would never find them.
Draco ultimately could not bring himself to kill the Headmaster, even after the arrival and encouragement of several Death Eaters, whom were smuggled into the castle by Draco. However, Snape — who had been called by Filius Flitwick to assist the Order of the Phoenix — arrived into the Tower and after Dumbledore pleaded with him, in a way that did not expose Snape's true intentions, cast the Killing Curse, killing the Headmaster and blasting his body off the tower to the ground below. Which resulted with Albus Dumbledore falling from the Tower after Severus Snape cast the Killing Curse on him.
Dumbledore and Snape had planned for Snape to kill him earlier. Dumbledore was going to die soon anyway because the ring cursed him, so he asked Snape to kill him instead of Draco to protect Draco from Voldemort's wrath.
Albus Dumbledore died at one-hundred and fifteen years of age.