Simplify the following text for a Grade 6 native English-speaking person.
Original: It is one of the world 's longest lasting string quartets , marking its 60th anniversary season in 2005 .
Simplified: The four players have not always been the same people : there have been some changes to the group , but it is one of the world 's longest lasting string quartets . In 2005 they celebrated their 60th anniversary .