I was just listening to a 6 min podcast by David Allen on how to teach teenagers GTD. Here's what to say to your teenagers.

Just write what you need to do down. Don't keep your commitments in your head.
Write down all those things you need to do to finish it.
Work out the very next action that needs to get done.
Once a week look at all the stuff you need to do and work out what you are going to do next.

We often, as a family, do a weekly review which gets our girls in the habit of this weekly ritual. Given how busy we all are it's vital to the sane working of our household.