Parents start off by talking about their children’s ages in terms of days, weeks, and months because when they’re young enough to be described that way, they’re changing every day, so just saying that they’re less than a year doesn’t give any kind of accurate picture of what developmental stage they’re at. But where does it end? Technically, whenever you want – no one can actually stop you from telling your child’s fourth grade teacher that they are 472 weeks old. They might give you kind of a funny look, though, while they try to do some mental math and figure out exactly what that means.
A good guideline for when people will stop knowing exactly what you mean when you talk about Baby’s months of life is the schedule of well-child doctor’s visits that start when Baby is a month old or younger. These continue at a decreasing rate until they are about 3, and after that they move into annual or biennial checkup territory. Until Baby is two, at least most parents will have a good idea of where they are developmentally based on their months of age, just like, when they were younger, they had a pretty good understanding of their age based on the number of weeks old, up until 3 months or so.