The biggest thing we could teach any young person is compound interest. If you start investing in your IRA or Roth IRA when you get that first job you will retire young and rich. Time is on your side. Just a couple thousand a year starting when you are 18 would do wonders for an early retirement.
Yep. I wasn't able to get to that point at 18. I waited until 25. We've been doubling our net worth about every 7 years. So at the end of our life, that 7 years delay between 18-25 will mean a HUGE chunk of change I could have left for my wife and kids. Fortunately, I'll still have half of that HUGE chunk.

But the thought of if I'd have just started 7 years earlier and the amount that would have been at age 70 is mind boggling.
Think about it.
If you have $20,000 and you double it, that's $40,000. That doesn't seem like much.
If you have $200,000 and you double it, that's $400,000. Hmm....
If you have $2,000,000 and you double it, that's $4,000,000. YIKES!
Yet, those are all the exact same percentages.
Get kids to understand compound interest!!! It can change their life.