That is a large question. For one, historians still have discussions of when the "Golden Age" was. Not all groups of pirates were the same, so who do you include? The Elizabethan sea dogs? The Caribbean Buccaneers? The Pacific raiders of the 1680s? The European-descended Red Sea pirates of the 1690s? The pirates who got spurred on by a combination of Bahama pirates in the early 1710s and the wreck of the Spanish Treasure Fleet in 1715 (who lasted until 1725/1726, little isolated groups popped over over the next few years, but didn't really make significant raids like the pirates previously)? The three most common groupings I see are the concentrated definition of about 1713-1726, the last group in my list that has the most famous pirates (because of circumstances in literature about pirates over the next about 300 years); 1690-1730 (dates are rounded off to nearest zero for convenience); and 1650-1730. It's hard to say who is right and who is wrong, though if we follow popular trends in literature and histories, the 1713-1726 and 1690-1730 definitions are the most popular in terms of defining the Golden Age of Piracy.

Why was there a conclusion of major pirate activity in 1725/26? Combination of several circumstances. The Navy was actively hunting down pirates, extremely few places would allow pirates into their harbors (a major blow was the closing of the Bahamas by the return of British authority in 1718 under Woodes Rogers), the decisive turn of practically all British colonial governments against embracing or turning a blind eye to piracy, and the decreased chances of pirates being able to make it out of piracy alive and with any plunder they've taken (it became more difficult to convince men to join in the 1720s, so a drastic increase in forcing men onto pirate crews resulted).

Hope that answers your question.