How Old Am I?

I put the wrong age on my Census form. I'm a year younger than I wrote down. I did get my birth date correct, so maybe they'll catch the discrepancy (I don't know for sure when I was born  -- I was born at exactly midnight and my parents had to choose which day my birthday would be on -- they disagree about which two days were involved in the choice).

I can't get my age correct in my head - when I'm asked I mostly get it wrong. Then, people think it's weird because, well, you should know your age. But I don't. Not sure why.