As you can see from the screenshot above, it will tell you how old the person is going to be when you mouse over their name (assuming you entered the year that they were born).
The main drawback to the module is that you need to enter the birthday information yourself on one long line:
The syntax for the Birthday Data preference is:
As you can see, each entry contains a date and the name of whose birthday is on that date. Entries are separated by semicolons. If the year of the birthday is unknown (or if you just don't want to admit it!), then it can be omitted.
For more third-party modules, check out http://www.googlemodules.com/, which is an unofficial repostiory for Google Modules.
Note: My Birthday Reminders module contains an Easter Egg, so if you don't like surprises (or you don't have a sense of humor), then I suggest you try a different module!