I agree with David and Affable Geek's answers: teens tend to think they are indestructible so you might not break through. Lord willing, they will survive long enough to remember your wise teaching.
A suggestion that I have which probably can't be applied to your situation is to take a look at John Piper's Desiring God. According to Piper, God isn't just offering eternal life, but also a joyful life. The subtitle, Meditations of A Christian Hedonist, really grabbed hold of me. He quotes from C. S. Lewis' sermon "The Weight of Glory":
If there lurks in most modern minds the notion that to desire our
own good and earnestly to hope for the enjoyment of it is a bad thing,
I submit that this notion has crept in from Kant and the Stoics and is
no part of the Christian faith. Indeed, if we consider the unblushing
promises of reward and the staggering nature of the rewards promised
in the Gospels, it would seem that Our Lord finds our desires not too
strong, but too weak. We are half-hearted creatures, fooling about with
drink and sex and ambition when infinite joy is offered us, like an
ignorant child who wants to go on making mud pies in a slum because
he cannot imagine what is meant by the offer of a holiday at the sea.
We are far too easily pleased.
Perhaps more useful is Blaise Pascal:
There once was in man a true happiness of which now remain to him
only the mark and empty trace, which he in vain tries to fill from all his
surroundings, seeking from things absent the help he does not obtain
in things present. But these are all inadequate, because the infinite
abyss can only be filled by an infinite and immutable object, that is to
say, only by God Himself.—Pensees, 113
So what I would focus on is that heaven is really an extension of all the great things that this life has to offer, only better (perfected) and without end. What Christianity in this life is all about is preparing us to be able to enjoy the bounty of the next one. Being able to communicate that Christian truth in a way that really connects with them is a question I don't really have an answer for, however.
I'll let the Angelic Doctor have the last words:
I answer that, As stated above (Q, A), the end is twofold: namely, the thing itself, which we desire to attain, and the use, namely, the attainment or possession of that thing. If, then, we speak of man's last end, it is impossible for man's last end to be the soul itself or something belonging to it. Because the soul, considered in itself, is as something existing in potentiality: for it becomes knowing actually, from being potentially knowing; and actually virtuous, from being potentially virtuous. Now since potentiality is for the sake of act as for its fulfilment, that which in itself is in potentiality cannot be the last end. Therefore the soul itself cannot be its own last end.
In like manner neither can anything belonging to it, whether power, habit, or act. For that good which is the last end, is the perfect good fulfilling the desire. Now man's appetite, otherwise the will, is for the universal good. And any good inherent to the soul is a participated good, and consequently a portioned good. Therefore none of them can be man's last end.
But if we speak of man's last end, as to the attainment or possession thereof, or as to any use whatever of the thing itself desired as an end, thus does something of man, in respect of his soul, belong to his last end: since man attains happiness through his soul. Therefore the thing itself which is desired as end, is that which constitutes happiness, and makes man happy; but the attainment of this thing is called happiness. Consequently we must say that happiness is something belonging to the soul; but that which constitutes happiness is something outside the soul.—Summa Theologica, Question 2, Article 7