Because life is impermanent even if you're young. There is no guarantee that any one person is going to be alive the next day. His Holiness wanted to emphasize impermanence is order to prepare the children for the afterlife.
Because even without the knowledge of Buddhism, the concept of death is universal and easily penetrates our mind. He talks about death in to healthy teenagers because the knowledge that nothing lasts but your state of mind is a good motivator for them to improve their mind.
His Holiness wanted to scare the teenagers using the horros of death. With this technique, they're less likely to commit harmful actions and realize that it will have consequences
Because attachment causes suffering and is the reason why we're still in samsara. His Holiness wanted to emphasize that they will take only their state of mind with them after they die, and nothing else. He spoke plainly about death because he wanted them to start letting go now before it causes them suffering at the end of their time.
They are not prepared to die
They are looking for buddhas and bodhisattvas for help
They've spent their lives relying on outside help i.e. shopping, addictions, money
They have nothing solid to hang on to
They rely on the groundless and inconceivable heart of bodhichitta
We can relax by simply being mindful of our panicking mind
We can relax by meditating on emptiness
We can relax by regularly applying the four powers of confession
We can relax by going to see the new movie that just came out in theatres this weekend.