When do it stop

My neighbor got so upset at me because on her son birthday card i wrote happy 3rd birthday but she teach her son that he is 36 months. I was like well excuse me I’m sorry I wasn’t aware. I always thought that the month thing stop once the baby is one. I did apologize and tell her I wasn’t aware but geez. When do people start to teach they kids they age instead of referring to them by months. I was not aware that some people do this until the kid is 4. Once my babies hit one I no longer did they age by months if they was 2 they was taught that they are 2 not 24 months. Even on doc papers I always wrote 1.2,3,4 etc instead of months. I guess I been under a rock because I completely upset her and I felt awful.