I think we can start to explain bitcoin by telling them that bitcoin is like other international currencies dollar, euro and many else. They will understand and you explain more about bitcoin.
* Yes, just giving them a simple explanation is enough, like say, 'Bitcoin is money, a type of digital currency which can be converted into any currency in the world and can be used to buy things in certain online sites.' After their interest is piqued, the rest will follow, they'll research and know about it on their own. Ways to earn, places to spend, and such.