I am going to say it started in late 1998 with a clean slate wiping out grunge influences and other mids 90s acts with Nsync and Spears and ended around late 2013-early 2014 with acts like Ariana Grande, Austin Mahone, Becky G, charli, iggy etc etc etc etc
2010-2011, around the time twitter and instagram took off. It's also around the the time people began to focus more on designing for the mobile user rather than desktop.
Hmm what? Gen Y and Millennials are the same thing. Everyone here over 16 is Gen Y. You're thinking of Gen X. Britney etc are late Gen X, Ariana etc are Millennials