@Pifanjr I can't say I agree with Diablo 3 not being simple, as you basically can play the game on the easiest difficulty and beat the game without really having much understanding of the game mechanics. I do agree that the skills trees could be a little confusing for a newcomer, but it's not really rocket science either, as you can just press on random skills and it would be sufficient enough to progress. The difficulty is scaling in both games, so I would think a newcomer would just kill stuff for a while, try on different stuff and learn what hits harder. The different colors should also be an easy indicator of what is good and what is not good.
Half-Life after thinking about it, I do agree. It starts out pretty simple, but yeah, there are definitely some elements in the game that would be very hard for a newcomer. Team Fortress is basically a kids game, so I don't really see it being much difficult for anyone to learn, regardless of experience. You will just die a lot at the beginning. WW2 Call of Duty games are pretty easy, at least while playing campaigns.
Another factor would be the age, as a teenager would be much more suited for learning computer games that need good eye-hand coordination, than an adult. Some puzzles would also be easier for older kids/teenagers as they are often much better at thinking outside of the box and trying out stuff that often adults would find to be not logical. This is why often older people need a little more time to get adjusted to learning video games, as they often don't understand why you should do this and that, like: "Why do I need to walk right when I can walk left." or "Why do I need to find a platform to open the door when I have a gun I could use to shoot it with".
I've thought some more about this and I don't actually think you can say a game is simple only because it's easy. I think it's better if a game is both easy and simple when using it to introduce someone to a new genre. I think Super Meat Boy is a good example of a simple game that isn't easy and Diablo 3 on the lowest difficulty is easy, but it isn't simple just because you can mostly ignore the complex parts.