If you were born on Feb 29 1980, how old would you be today?
I propose that you could choose from one of two ages, depending on what was most beneficial to you at the time.
1. You could be 22. It has been 22 years since your birth.
2. You could be 5. You have had five birthdays.
(Note: I don't think 20 is an acceptable option)
Because of the fact that you could be either 22 or 5, depending solely on your discretion, I would actually submit that perhaps the person whose age is hardest to tell is the one born on Feb 29, because they can decide how old they want to be. Plus, if you didn't know their birthdate, you'd never guess that the person in my example was 5.
Man, I wanna go see the Pirates of Penzance.