The gravedigger reveals this in his conversation with Hamlet: (5.1.56-100). All the edited texts take their wording from the Second Quarto edition:

Hamlet: …How long hast thou been a grave-maker?

Gravedigger: …I came to’t that day that our last king Hamlet overcame Fortinbras…It was that very day that young Hamlet was born…I have been sexton here man and boy thirty years….this skull hat lien you i’th’earth three-and-twenty years….this same skull, sir, was, sir, Yorick’s skull…

Hamlet: …He hath borne me on his back a thousand times…those lips that I have kissed I know not how oft….

In the First Folio edition, though, the line reads "I have bin sixeteen heere..." (See Chapter One, and many contemporary "sixeteene" usages here.)

In the First Quarto edition, "I have been sexton here man and boy thirty years" is missing, and the gravedigger says instead of 23 years, "here’s a scull hath been here this dozen year." : (Q1: 3361) Louis Vuitton Bag with Zips Louis Vuitton Black Luggage Bag Louis Vuitton Mens Collection 2011 Louis Vuitton Accessory Handbags Louis Vuitton Epi Leather Case Louis Vuitton Epi Leather For Sale Louis Vuitton Epi Leather On Sale Louis Vuitton Mahina Leather Black XS Handbag Vernis Louis Vuitton Shoes Louis Vuitton Outlet Store in the United States Louis Vuitton Outlets in Amsterdam Netherlands Louis Vuitton Outlets in Athens Greece Louis Vuitton Replica Luggage Set Replica Louis Vuitton Leopard Bags Louis Vuitton Offer Discount Louis Vuitton Boutique in Barcelona Spain Louis Vuitton Boutique in Brazil Louis Vuitton Valencia Outlet Louis Vuitton Wallet For Women Louis Vuitton Wallet Mini