I think we can start explain bitcoin by tell them that bitcoin is like other international currencies dollar, euro and many else. They will understand and you explain more about bitcoin.
This is a good start, personally i had trouble grasping the ideology and aspect of bitcoin
when i started. It took time and persistence. You have to make them understand through
the simplest and approachable method. Teaching them about saving money and the
different currencies would be a nice beginning, for teenagers you refer them to youtube
and to the extensive articles and webpages dedicated to bitcoins. Encourage them to read
and to ask questions, bear in mind this requires for you to have a huge amount of patience.