] He's at university, moody, and into existentialism, so he acts like a 20-year-old student. But the scene with the gravedigger seems to imply that he's at least 30. That just doesn't seem right to me; Hamlet's actions seem to reflect the attitudes and behavior of a young man.

I read Harold Bloom's book about Shakespeare's plays last year(he's a noted teacher somewhere in New York). He talks about the age confusion in the chapter on Hamlet. His theory is that the Hamlet we know and love is a revision of an earlier version, which accounts for the origin of the confusion. He thinks that Shakespeare was so caught up in the story he was trying to tell, and needed the different ages to make it work from an emotional standpoint, that he didn't care if it was inconsistent. I believe that his theory takes into account that in an earlier version, Hamlet might have been gone to England for a much longer period of time.