I generally get a range between 23 and 29, which is reasonably accurate I guess, but about a third tell me that I'm a woman. tbh I'm pretty good at looking like someone with gender identity issues so I can't complain.
EDIT: Also, if the photos aren't taken head-on the results will be shit. I think they also manage to take face creases into consideration, meaning if your mouth is open or otherwise stretching your face, it will add several years. The neutral photos I take with less contrast always get me the lowest age.