Hm. I would guess younger people are more likely to have their age on their fb profile - to make this more than just a random collection of data, you'd need to check that as well. Also, is there a correlation between age and whether you include your social networking profile on your stack overflow profile in the first place?

Would be interesting to know how he did it in detail.

Twitter does not contain age, neither does LinkedIn (or at least not show it) And in facebook you need to jump through loops like user_birthday and friends_birthday[1]. I don't know how to get the birthday of 3rd parties.

[1] https://developers.facebook.com/tools/explorer?method=GET...

You can input your age on a Stack Exchange profile.

ok that was too easy.

I think that's an issue in his study. It means that the age is restricted to the age range of Facebook users (and maybe to the younger FB users, who are more likely to give their date of birth).