Complex sentence: Its founding editors were Jenette Kahn and Jim Robinson, who were adults; however, later in its history, it had a teenager, Denise Yuspeh (age 15) as its managing editor.
Split into simpler sentences: Its founding editors were Jenette Kahn and Jim Robinson, who were adults. However, later in its history, it had a teenager, Denise Yuspeh (age 15), as its managing editor.