Babies Age

I just went and visited my cousins newborn baby. She was born at 3:31AM this morning. On the ride to the hospital, I asked my parents how old a baby has to be before you start saying the age in years instead of months. For example, I say that once they hit 1 year, you should start saying the baby is one year and 3 months, not 15 months. You can say 6 months, 8 months, and 10 months. After a year, say one year and blank months. My parents say it should be two years. I think it's just weird to say, "My baby is 22 months." No, he is 1 year and 10 months. As long as you don't go past two years. "My baby is 36 months." Really? No, actually, he's three years old. What do you think?