I've always assumed Hamlet was in his late teens-early 20s, but all the actors I've seen play him are aged mid-late 30s. Any theories why?

Hamlet is the lead in a very famous play. Audiences will expect to see a well known actor in the role, and an established, well known actor is much more likely to be in his thirties than in his teens or twenties.

According to the text of the play Hamlet is actually thirty years old - the grave digger says he has been a sexton for thirty years, and started his job on the day that Hamlet was born - which seems to settle the matter.

Tina is right - Hamlet was 30 years old.

And most actors who get to play him in major productions are in their late 20s to mid 40s because it's a huge role, with more lines than any other role, and it takes a lot of experience and huge talent to do it justice.

young characters are very often played by older actors. probably because available actors the age of the character wouldn't generally be as good for lack of acting experience

you are watching plays and films in which the actors cast are in their mid to late thirties.