- You should check if `formattedDate` is actually a Date object before using it (and change the variable name, since it sounds like it contains a formatted string).
- It's strange, if not wrong, to consider months to be exactly 30 days. 
- You unnecessarily calculate `Math.round(difference_ms/ONE_DAY)` twice.
- `yearAge` is a bad name for the variable at the beginning, since it doesn't contain anything concerning the year. And you reuse the variable later (then the name fits).
- `var today = new Date(yearNow,monthNow,dayNow);` is wrong (and unnecessary), considering you just got `monthNow` from the current date and added `1`.  Fortunately you don't actually use it.
- Testing for the full year to be below 100 is unnecessary. Only `getYear` returns such values.
- It probably would be better just to compare the day, month and year of the birthday and the current day, and do those calculation of the age in days and months only if needed.