Ariana - Close to 0. The Moonlight era just started, and most of her fanbase is on My Everything. She's still even on her tour for My Everything.
Zedd - 33% are stuck in the Clarity era. idk on this one tho
Gaga - 10%, most are hyped about things from this year
Imagine Dragons - like 90% are stuck in the Night Visions era

JT - 100% because he's stuck in 2013 himself