Why didnt Nelson Mandela become president until he was 76 years old?

Because before the 1991 end of Apartheid, blacks in South Africa did not even have the right to vote, amongst many other rights they didn't posses. For the previous South African governments, Mandela, as the leader of the black opposition (of the ANC), was considered a dangerous terrorist, and he was imprisoned for his attempt at ending discrimination. After the end of the Apartheid regime, he was elected in the first free elections of South Africa.