There were actually two surveys conducted. The first had a sample of 1,843 men and women aged from their teens to 49 years old, and the second involved a subset of 335 people from the original sample (I think – it is not too clear) who had identified themselves as otaku in Q1.
Although the demographics are confusing, there does seem to be an indication that the eldest child is an otaku. There is perhaps a reason involving Japanese family structure, but what that might be I have no idea! I welcome any wild speculation from my readers!
These holy grounds are the real life locations represented in various anime and manga; for instance, here is their entry for the hot springs and hotel in Sprited Away/Sen to Chihiro no Kamikakushi.