It's not always accurate.
Trying out the site, I uploaded 2 different photos of myself to see what age the algorithm came up with for me. The age difference in the two analyses was more than ten years. To top it off, the photo where I thought I looked older was the one that the website said I looked younger.
I had an idea that the absence of hair on my head might be skewing the outcome. I decided to take one particular photo and with the power of GIMP, I added hair and uploaded the results to how-old.net to see what would happen.
The results follow: