I think the 14 year olds were attracted by a sense of lawlessness, most visible in the long, highly editorialized titles and the trollish comments.
It wasn't so much general interest articles that signalled the decline of Reddit as partisan political ones. I.e. articles that didn't merely involve politics, but were instances of it.
What ruined Reddit was (a) an influx of 14 year olds, who (b) were allowed to continue to behave as they had on Digg or wherever else they came from.