Simplify the following text for a Grade 5 native English-speaking person.
Original: The most precise estimate of the universe 's age is 13.73 Â 0.12 billion years old , based on observations of the cosmic microwave background radiation .
Simplified: The best good guess of the Universe 's age is 13.73 Â 0.12 billion years old , based on what was seen of the cosmic microwave background radiation .