[coward123]

My son, also 13, just went through a program at school where they built a basic website from scratch, then did some python, then some java. They had some kind of teaching environment, a bit like a code.org or Swift Playground. When I would look over his shoulder to help, he was certainly coding an answer, but the way it was set up was so contrived that I can't really say he learned any of those languages. He learned a set of contrived functions nominally using those languages that he assembled in order to get the right answers.

I share this as preamble to why I recommend Swift Playground...it will get them going and be somewhat self directed. They will feel like they are actually achieving a goal rather than just typing text. And, if they take it to the next level they will be able to transfer their knowledge into "real" apps.

Second to this would be Python, because it's a great teaching language. That said, I think it's about the curriculum you put in front of them even more than the language.