Best way of explaining is using the gold analogy.
But I think explaining it to a person in his/her early teens will be quite diffiicult. Best to wait till the person reaches late teens and then explain
To be honest I didnt know what does the gold analogy, but in terms of explaining what Bitcoin is it depends on how deep that a peeson knows something for this kind of platform the idea of supply and demand can be really helpful for a person who knows this one, and it was tackled during secondary level and I think this is aint suit for children.