A longtime Google executive who played a key role in the company's creation is stepping down as YouTube's CEO. Susan Wojcicki had spent the past nine years running the video site that has reshaped entertainment, culture and politics. In an email publicly shared Thursday, the 54-year-old Wojcicki said it was time to "start a new chapter." Her departure comes at a time when YouTube's ad revenue is slumping for the first time since Google's corporate parent began disclosing the site's financial results. Besides being a top YouTube executive, Wojcicki also served as Google's first landlord in 1998 when she rented her garage to the company's co-founders.