"officially", by JC regs, Aero (Lady Brave Heart) was 7 on New Years Day. However, her real birthday is in April. Katy gives her two birthdays since it's double the excuse to buy more horse stuff.
Yahzi is a grade with no known history - so I have no idea about her, so we just use the day we brought her home as her "birthday" (because all our animals get to have a birthday, lol)
The Arabians that I have owned all aged on their actual birthday. The hubby's QH is technically supposed to age on Jan. 1st, but I told DH since he isn't shown, raced, or anything else his bday will be his bday, I don't care what AQHA says.