Simplify the following text for a Grade 9 native English-speaking person.
Original: Old Kingdom is the name given to the period in the 3rd millennium BCE when Egypt attained its first continuous peak of civilization in complexity and achievement â `` the first of three so-called `` Kingdom '' periods , which mark the high points of civilization in the lower Nile Valley ( the others being Middle Kingdom and the New Kingdom ) .
Simplified: The Old Kingdom is the name for Egypt during the 3rd millennium BC when the civilization of Egypt had its first peak . It was the first of three so-called `` Kingdom '' periods , which mark the high points of civilization in the lower Nile Valley ( the others being Middle Kingdom and the New Kingdom ) .