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Depends on the person you are teaching and the area of that range.

A 12-14 year old that WANTS to should be able to handle just about anything, but he has to pull it in his own time, you can't really push complicated concepts to young people (or for the most part anyone).

I'm listening to iWoz at this point (which seems to be targeted at that age group and would be quite inspirational), by that age he was putting together some pretty advanced circuitry--but his father only ever answered questions, never handed him new concepts he wasn't ready for)

Or he may be completely disinterested and there is nothing you will be able to do about it.

Kids can be really impressed with something simple though. If you found some game he liked and helped him recreate it (even on a very superficial yet still graphically similar level) you may really get him going.

Or, even better yet, if you found an existing open source game he might like, let him play it then show him how to make little modifications you might be able to get him excited. (Modifications always seem to be the best way to get started)