Simplify the following text for a Grade 6 native English-speaking person.
Original: Originally named as the `` Royal and Pontifical University of the City of the Kings of Lima '' , San Marcos was chartered on May 12 , 1551 by a royal decree signed by King Charles I of Spain , which makes it the oldest officially established university in the Americas and , as such , one of the oldest universities in the world .
Simplified: It was founded on 12 May 1551 . It is the first and most important university of Peru and the oldest university of the Americas .