Simplify the following text for a Grade 4 native English-speaking person.
Original: They grow very quickly , at more than a metre a year , and can reach 65 m in 50 years , with an average life-span of 400 years .
Simplified: They grow very quickly , at more than a metre ( 3 feet ) a year , and can reach 65 m in 50 years . They live for an average life-span of 400 years .