Chris Breen of Macworld fame posted a handy tip on how to get birthdays for the 75+ among your family and friends to show up on the iPhone. (Seems the iPhone OS isn't happy with events that repeat more than that many times, hence no birthday love for the seniors).
The solution is to create a separate repeating event in a different iCal calendar for your well-seasoned friends and family members that begins this year. Unless they live to be more than 150 years old, you should be set.
Check out his full post for details on managing birthdays on OS X in general, and syncing them over to iPhone.