Simplify the following text for a Grade 9 native English-speaking person.
Original: In 1895 , John Perry , a former assistant to Kelvin , produced an age of Earth estimate of 2 to 3 billion years old using a model of a convective mantle and thin crust .
Simplified: In 1895 , John Perry produced an age of Earth estimate of 2 to 3 billion years old using a model of a convective mantle and thin crust .