3/4 of a Year Old

So when do you start refering to a baby's age in years instead of months? I am not sure what is proper ettiquette so I decided to try it out at 9 months. Beckett had his 9 month appointment on Monday. We were very excited to find out that he did not have to get shots at this appointment. He was, as always, very active and not very cooperative but he got a clean bill of health. I was surprised to hear that he was only 9th percentile in weight and 75th percentile in height. Is he really that skinny? Here is a picture of him in his halloween costume (since I didn't post about that), you decide.