Complex sentence: Since "SpongeBob SquarePants" made its debut in 1999, the show had flourished into Nickelodeon's 2 children's program, after "Rugrats", upon which nearly 40 percent of its audience of 2.2 million were aged 18 to 34.
Split into simpler sentences: Since "SpongeBob SquarePants" made its debut in 1999, the show had flourished into Nickelodeon's 2 children's program, after "Rugrats". Nearly 40 percent of the show's audience of 2.2 million were aged 18 to 34.