Hamlet's age is debateable.  When Hamlet is talking with the gravediggers in Act 5, sc. 1, the first gravedigger says Yorick has been dead for 23 years.  Yorick was a court jester when Hamlet was a boy so that would indicate that Hamlet is about 30.  At the beginning of the play however, we are told he had been away at college when his father died and that was just about two months ago.  Most college students are under the age of 30.  The question of time passage in the play is vague, too, remember.  When they are watching the play within the play, Hamlet tells Ophelia that his father's been dead "...within these two hours," and Ophelia tells him it's been four months since his father died (Act 3, sc. 2).  We get the feeling as we read the play that things happen at a steady pace, but that's not necessarily so, for example, it would have taken some time for news of Polonius's death to reach Laertes and then for him to get back to Denmark. I explain all that to suggest that maybe when Hamlet leaves Denmark and is taken aboard the pirate ship, we don't really know how long he was gone.  Years may have passed from the beginning of the play to the scene in the graveyard which would put Hamlet at about 30.  Also, we can't be certain that the lines were all written correctly from memory and that is how most of the lines were recorded.