Okay, lets look at Starcraft.
I doubt that if I put many of the Dads I know (over 40) in front of Starcraft, they would be able to pick up and play the game or even make the choice to buy the game in the first place regardless. So that cuts out the mature market.
Under 12's might be able to understand the game better but again, given the current market, would the PC be the first platform of choice for games? Wouldn't they prefer their brand name consoles? Goodbye children's market.
So that leaves the teens and young adults. Out of this group, again how many would have their PC as their platform of choice to play games on? If this was a cutting edge game, how many would even have the hardware to run the game well? Also, how many PC gamers would buy a RTS game especially if they are 'casual' gamers?
Immediately you see market shrink and predict that the most common players for Starcraft 2 would be teens/young adults who are PC RTS hardcore players.