You just have to look at their actual behavior. I do sweet 16's... they ALL ask me "you don't play the entire song do you". But it's more than that, and not just teens. Here's examples:
- when multiple people are in cars, someone is changing the song every minute now, instead of letting it play out. And not because "they're tired of it"... but it's more like a song plays for a minute or so, and someone says to change it for whatever reason.
- people are downloading podcasts with premade mixes from dj's who are incorporating only 60-90 seconds of a song.
- when teams are warming up at games, they're playing many songs during that warm up time. Ps many stadiums are hiring dj's to mix during downtimes.
- the radio itself during peak driving times are having live dj's mix as many songs as possible during that hour frame
- people listening to their music at the gym are changing songs with every single set they do.
- people for private home events are playing soundcloud and youtube mixes that have 30-40 songs mixed in an hour.
Everyday behavior is changing when it comes to music. Whatever your opinion is on how they'll behave later in life is irrelevant to the dj whose focused on booking jobs now. WHereas club dj's in the past did this, and only those that went to clubs were exposed to this... now this behavior is everywhere and commonplace. It wasn't iphones that changed this... it was iphones in combination with spotify and youtube that gave people access to every single song out there, as well as the radio changing it's behavior. Yes, there are some songs that are so big that people want to hear it play out, and some classics where every verse is needed... but the majority of the newer tracks are repetitive. Likewise, as chris said, it's also being aware of who your crowd is. If it's an older crowd, or a crowd that isn't as into music and into the latest and newest, or going out a lot... cutting songs short will turn them off.