Remember, they may be gathering more info as more people give their real ages, and can tune-up the accuracy. Also, possibly collecting other information and this is just a way of getting that.
Guessed 32
I’m 62 and I did not go as fast as possible.
Not impressed.
Guessed 31, I’m 49. Did this on my MacBook Pro with the touchpad, and I expanded and enlarged the Firefox display settings until the window could not be displayed in its entirety – still got a “too small” warning.
I’m 47.
Did it last night at home on the home PC right handed got 42.
Did it again at work left handed and got 37.
I’m right handed but at work I use the mouse in my left hand so I can use my right hand for writing, using the phone, other stuff at the same time.
Guessed 42; actually 30. 
Guessed 30, actually 36. And I kept getting “page not available” errors as I clicked through each stage.
I’m 37 and it guessed I was 28. I’m using a trackball; I don’t like mice.
It guessed 41 and I will be 41 in a few weeks. That was pretty freaking amazing!