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My son had watched the Starcraft 2 trailer with me and is a big fan of real time strategy games so I thought I would purchase him a copy of the original Starcraft and Broodwars which would play fine on his older machine. He had been playing Warlords 2 & 3 and has played Glest and some other Real Time Strategy games. At six year old he plays far more complicated games than I was playing at ten years of age, and his reading and ability to figure out these type of unit and resource management games is very good for his age.
I do think RTS games teach him to manage resources, count, calculate and develop strategy and help in critical and analytical thinking. The fact that most RTS games have a fairy amount of reading in them also helps with his vocabulary and reading skills. So my son dove into Starcraft with abandon with me assisting whenever he ran into trouble and I got to fondly remember Jim Raynor, Tassadar and the other characters in the original Starcraft universe.