Absolutely any survey where respondents select themselves is a questionable data-source. But some data is better than no data. If you think these results are totally unreliable I'd like to point out that professional surveys tend to show MMOs and RPGs both have audiences that skew older than gamers in general, and gamers and generally aren't quite as young of an audience as some would expect anymore.
This data falls in-line with the general trends you would expect to see in a title like this. So I'm guessing it's fairly accurate.
I wouldn't say it unquestionably prooves that 50%+ of this game's audience is 30+. But I'd say it proves that is more likely than not. And it unquestionably proves a large portion of this game's audience is older.