Yeah, somebody at the IRS was mathematically challenged when they came up with this.  Your birthday is actually the day before you were born.  Go figure.  See page 2, table 1 (2018):

https://www.irs.gov/pub/irs-pdf/p501.pdf

" If you were born before January 2, 1954, you're considered to be 65 or older at the end of 2018"

It applies to other things too but this was the first thing I found in google.

Rick