Simplify the following text for a Grade 7 native English-speaking person.
Original: During the late 1950s and 1960s superhero-revival period known as the Silver Age of Comic Books , he was a founding member of the Justice League of America . In the 1990s Modern Age of Comic Books , Aquaman 's character became more serious than in most previous interpretations , with storylines depicting the weight of his role as king of Atlantis .
Simplified: During the Silver Age of Comic Books , Aquaman was a founding member of the Justice League of America .