Simplify the following text for a Grade 9 native English-speaking person.
Original: In April 1991 , Linus Torvalds , a 21-year-old student at the University of Helsinki , Finland started working on some simple ideas for an operating system .
Simplified: In April 1991 , Linus Torvalds , then 21 years old , started working on some simple ideas for an operating system .