How can you emphasize to youth that everything they do will haunt them for the rest of their lives?

Why would I emphasize a lie?

Your statement is incorrect. 

A few things I did haunt me, the majority are fond memories 

You can't.  Each of us has to learn from our own mistakes.

You can`t....because old age to them, is a million years away and will never happen to them. 

THEY feel immortal, and of course have no concept whatsoever of how it feels to be elderly and wiser and have regrets about past events. That`s life I guess!!! JMO.

Children rarely listen to advice from parents.  The best way that any of us ever learnt anything of any value was to get it wrong the first time. It will not be vastly different for todays young people.  Experience teaches us far more than any formal lessons ever do.

i dont think you can, they'll have to learn that on their own

By telling them about some of the stupid things you did and how you always cringe when you remember them.