Age Calculator app using HTML ,CSS, javascript and jquery

Things i find difficult in this project are to conditionally display the error like "This area is required" and "must be a valid date"

The area im unsure about are im still uncapable to display mutilple errors at once like if only Month and Year are empty i cant display "This area is required " in both of them without making it complex

I'm thankable to anyone who can suggest how to tackle these problem