Screen Age is the age of actors which they list in their acting profile. Screen Age isn’t the real age of an actor instead, it is a factor for the director as to whether the actor is capable of playing the character of that age.
For example, you can take actor Rajkummar Rao who is 36 years old, but he easily plays characters in the age group of 25-35 on Screen. So in such a situation, we can say that the Screen Age of Rajkumar Rao is 25-35.
NOTE: Always update the Screen Age in your acting profile like 20-25, 30-35, etc!!