Think you look young for your age? Microsoft has a new tool that could either give you an ego boost or -- more likely -- burst your young-at-heart bubble.

The How-old.net tool, unveiled Thursday at the company’s Build 2015 conference, lets users either upload a photo, use a stock photo, or do a Web search for photos. It then analyzes the faces in the shots and takes a guess at the ages.

The tool is far from 100 per cent accurate – beards and facial stubble seem to throw it off, for example – but it generally lands in the right age ballpark.

At least for the most part. Twitter users have been having fun over the last day pointing out how very wrong the tool's guesses can be.

According to one of Microsoft’s blogs, the tool was developed while company engineers were “playing around” with new face detection software. They asked a few hundred people to try the tool out and tell them what they thought and were taken aback by how many people responded.

"…We were expecting perhaps 50 users for a test but – in the end – got over 35,000 users and saw the whole thing unfold in real time," the bloggers write.

A warning before trying it: the ‘Terms of Use’ page linked to the website says users are granting Microsoft and its affiliates permission to use submitted photos for “business purposes.”

That includes allowing them to, “distribute, transmit, publicly display, publicly perform, reproduce, edit, translate, and reformat your Submission; to publish your name in connection with your Submission; and to sublicense such rights to any supplier of the Website Services.”