20 and a half !!

I recently went to see a doctor for a check up. The conversation with the receptionist went a little something like this:

-Receptionist: Are you at least 18 or older?
-Me: Yes.
-Receptionist: Please sign this form.
-My Mother: [Proudly declares] She's actually 19!
*Receptionist gives a half smile...*
*I look at my Mother waiting for the joke...*
-Me: No, Mom. I'm actually 20 (and a half).

I won't tell you her age, seeing as she just had a birthday and may be a little sensitive about getting older... but you would think it would be really easy to remember my age considering that we're exactly 30 years apart!