[joelmaat]

The same way you'd teach anyone else, as there isn't any advanced Math or vocabulary necessary.

Does the teenager have a knack for it already? I could've been coding before I was 10 years old and it would have been no more difficult for me to learn.

Making it a (fun) game is a good idea. Or you could have them come up with something they've been wanting to do, then use the process of creating that solution to introduce them to the concepts behind programming, and to how to apply them. Just have them work out how they'd go about creating that thing they mention, then show them how they are essentially programming already, and that the language they are learning is just a different way of expressing what they are already saying.

If they become curious or interested, then they'll spend their own time fighting with the code, and that will allow it all to sink in in a way they'll understand.