As the root verses read: Whoever is born possesses the phenomena of impermanence and death. Thus, wherever one goes, wherever one is born, then from birth, without depending on measuring by the numbers of more or less beings, impermanence is the conditional phenomenon of death. As the Great Pandit, Ashvaghosha, said:

Whether on earth or else in the heavens,
Once born, was there ever anyone who did not die?
Did you ever see anyone?
Did you ever hear of anyone, or did you have any doubt?

Thus, as said, here in this universe, has one ever heard of any being who never died and is still living at this time? Did anyone ever see any such being with their eyes, or ever hear about them, or have any doubt? There is no escape from death, so therefore, such decisive questions are asked by the learned one. However one thinks, whether considering the outer container of the universe or the inner essence of beings, it is all conditional phenomena that is destructible. From The King of Samadhi Sutra:

When this world of cyclic existence occurs,
By occurring, it diminishes and nothing is left.
Likewise, as it was before, the future will be like that.

Thus it is said.
Although thinking it is true that taking birth leads to death, one might still speculate, according to the strength or weakness of one’s health and whether one is old or young, that one is young and healthy, and for this reason may assume that one will continue to live for a while. It is impossible to predict that. As it says in The Voice of My Root Guru, the Emanation of Samantabhadra: 

Some infants still crawling meet with death. Some youths meet with death. For some, death comes when they are old, frail, and decrepit. Others may not even have the opportunity to take medicine or to try to protect themselves from sudden, untimely death. Some are ailing for so long that their bodies are stuck to the bedsheets while their dead eyes continue to gaze longingly at the living, with only skin covering their bones, and then they die. Some suddenly die of a stroke without even finishing their meal, conversation, or work. There are even those who die by suicide. There are many conditions for death. The power of life is weak in comparison, like a butter lamp in a windstorm. Even today at this very moment, death could suddenly occur, so that by this time tomorrow, rebirth as an animal with horns or a beak could have already happened. The time of death is extremely uncertain, and so is the future place of rebirth. It is important to be concerned about this.

Since the circumstances of death are so uncertain, it is important to have confidence in the immeasurable teachings that are given. As it says in The Speech on Impermanence: 

For all who are born,
Death is staying before them.
Since even I have not transcended that, Dharma must be practiced from today.

Thus it is said, and, from The Sutra of Parinirvana:

For whoever harvests fields, the best is to have found a field with crops. Among all impressions, the best is the perception of impermanence and death. Why? Because the perception of the three realms will be diminished.

Thus it is said. Then, in this case, the meaning of reversing the mind that identifies with the three realms is as follows. According to the Hinayana vehicle, knowing the passions and their karma are exhausted so there is no cause to be born again is the attainment of the Arhat state. According to the great vehicles of cause and result, the mind of the desire realm is exhausted in the wisdom mind of Nirmanakaya, the all-encompassing compassion of empty bliss. The mind of the form realm is exhausted in the wisdom mind of Sambhogakaya, the spontaneous presence of empty radiance. The mind of the formless realm is exhausted in the sphere of the wisdom mind of the empty awareness of Dharmakaya. In this way, the sublime permanent state of deathlessness places one in the state of the Victorious Ones of the Three Kayas. The meaning of this is in The Sutra of Revealing the Three Kayas:

Dharmakaya is the permanence of the essential nature as it is.
Sambhogakaya is constant permanence.
Nirmanakaya is uninterrupted permanence.

Thus it is said. Also, in the Uttara Tantra, it says:

The lord of Dharma annihilates the demon of death,
And since this is unborn, it is the permanent protector of the world.

Then, from Uma ]ukpa:

Body of peace, the wish-granting tree, ever radiant,
Like a wish-fulfilling jewel with no conception,
Permanently endures in this world until all beings are liberated.

Thus, Dharmakaya and Sambhogakaya are permanent. Nirmanakaya is both pure and impure, according to the purity or impurity of the vessels of water of various beings’ capacities, which reflect clearly or unclearly the image of the moon that appears in the sky. Just as the actual moon does not really enter the water to become self-characterized within water, the nature of phenomena is likewise beyond the nature of permanence and impermanence, and so is called the permanence of the uninterrupted continuity of Nirmanakaya. Therefore, since the result of meditating on impermanence is the attainment of the deathless, permanent, sublime state of the Victorious Ones of the Three Kayas, be diligent

A Cascading Waterfall of Nectar, Pgs 39-42
by Thinley Norbu
Shambhala Publishing