I remember years ago, if you went to a carnival, there would be guy standing by a scale who, for a dollar, would guess your age within two pounds or your weight within three pounds. If he missed, you would win a stuffed animal. He would almost always be right. But if he was wrong, he only had to give out the toy, which probably cost him less than a dollar in quantity.
Now, I haven't seen an app or web site that guesses your weight (hey, there's an idea for a startup), but there is a site that guesses your age. It is called HOW OLD DO I LOOK? and the web address is How-Old.net.
What you do is type in the name of a person who's picture you want to find on the Internet (like maybe yourself), and it comes up with a group of pictures. Click on the picture you want to check, click on the Use This Photo link, and a page appears showing the estimated person's age.
If you can't find the picture you want on the Internet, you can upload your own photo from your computer.
It seems to err on the side of younger for older people and older for younger people.
An interesting idea, which should trigger other similar startup ideas for you.