Simplify the following text for a Grade 4 native English-speaking person.
Original: Frederik Willem de Klerk ( born 18 March 1936 ) , often known as F. W. de Klerk , was the seventh and last State President of apartheid-era South Africa , serving from September 1989 to May 1994 .
Simplified: De Klerk was born in Johannesburg , on 18 March 1936 . He was the president of South Africa from 1989 to 1994 .