"If I'm going to be perfectly honest. Which is rare, in this matter."

Perhaps one might question why it is that he can't be honest about it. That he prefers to lie, if not to be outright secretive and evasive. A way to escape from the matter that he actually doesn't know, maybe. Distract himself from the fact that he has no idea how long he's actually lived.

How long he's actually struggled.

Or maybe it's simply because there are so many years of his life that he isn't proud of, and he'd rather not think too much about all of those.

This is the most honest he's been for a very long time, about his age.