Chronological Age is a very easy to use Dashboard widget that allows you to calculate your chronological age.
Enter your birthday, and today's date and see what the chronological difference is in days, months and years.
To install the Chronological Age widget, when the download is complete, unarchive it and place it in /Library/Widgets/ in your home folder. Show Dashboard, click the Plus sign to display the Widget Bar and click the widget’s icon in the Widget Bar to open it.
To learn more about dealing with the Mac OS X Dashboard you can easily access the Apple support page HERE.