Because according to the ESA's 2011 Sales, Demographic, and Usage Date
, only 18% of all gamers are under 18 years of age.
Other interesting stats include:
72% of American households play computer or videogames.
29% of gamers were over the age of 50 in 2011.
53% of gamers fall between 18 and 49 years of age.
33% of gamers say that gaming is their favorite entertainment activity
You should follow the link and read the whole thing as it's fully of interesting information regarding gamers and their (our) habits.