You know your old when...

[Fabo Craven]

When the person lighting the candles gets tired.
when they light the last candle, the first ones burned all the way to the cake
post other ideas here.

[Kisanzi]

When you no longer have real Birthday parties, but rather a small gathering of friends who eat dinner, give you "gifts" and then call it a night.

When you get nothing but gift certificates as presents.

When you go half the day like nothing is different until somebody reminds you what day it is by telling you "Happy Birthday".

NOTE: All of these are based off of personal experiences...yes, I forgot my own birthday last year...*sigh*...