I dont have a code reference, but there are tons of articles out there that say if your birthday is on Jan 1st youre considered that age on Dec 31st....like this one.
https://apps.irs.gov/app/vita/content/globalmedia/eic_without_a_qualifying_child_4012.pdf
♪♫•*¨*•.¸¸♥Lisa♥¸¸.•*¨*•♫♪
