Good idea. Would have preferred a count in months, days, hours, minutes, seconds and ms, but this will have to do - I did not find anything like this.
But can you be more, well, descriptive, in your description? I had to guess what this is. I didn't understand it at first. How do you expect people to put their DOB in? How do you expect people to download this if they don't know what this is for?
E.g. Replaces new tab page with Motivation.
Once installed, enter your date of birth and then, every time you open a new tab, a live calculation of your age will appear.