Microsoft has released the free 'How Old?' webpage that guesses both the gender and age of the picture's subject.
I tried it out and got mixed but fairly accurate results, ie:
Indeed, I am both male (indicated by the symbol next to the age) and 36 years old. I tried it with a mix of images, and it was fairly consistent. It never got my gender wrong (thanks goatee!) and it generally guessed me at 37 years old, which is pretty close.
However, not everyone experiences good accuracy. From comments on Microsoft's Face API page:
The girl above is obviously not 56. Nor a man.
And this girl is clearly not a 74 year old man, either.
Many users report similar errors, so it is not foolproof by a long measure. Images with groups of people don't appear more prone to error, but some faces are consistently tough to 'guess'.
However, the analysis works on faces previously unregistered with (unknown to) the database, so it is interesting.
Demographics analysis has been a niche feature of analytics providers for a while, ie: 3VR Demographic Analytics (Gender / Age), so Microsoft's implementation is sure to attract interest.
Give it a try and let us know how accurate it is for you.