However, that group is fairly equally split (45% Yes/44% No). I reckon they underestimated how strongly popular culture, which is focused on London, influences the 16/17 age group.
Support for votes at 16 is long-standing. As is support for the same measure by most UK parties.
On the subject of 16-24s being 'equally split' - note the same could be said of other age-groups (apart from the more strongly No cohort old people), so 16-24s being the same doesn't say much for a resurgence of Britishness amongst the young.
Edit: Whoops this was meant to reply to cstross below as a general point about unpredictable youth voting intentions not the intentions of Scots youth in particular.