Complex sentence: By age 12, he was writing fan letters to primatologists; he attended John Dewey High School and by that time, he was reading textbooks on the subject and teaching himself Swahili.
Split into simpler sentences: By age 12, he was writing fan letters to primatologists. He attended John Dewey High School and, by that time, he was reading textbooks on the subject and teaching himself Swahili.