Well they've made an error somewhere because in that same paragraph they say "Thirty five percent of game players are usually under eighteen years, and 65% of game players are over eighteen years old." which does not make for an average of 18 unless the ones on the under-18 side are radically lower.

Also go back and read Vivi22's posts again: The fact that there are some outliers among a large number of surveys doesn't actually nullify the value of those surveys!