I don't think 12-20 year olds are that mercurial, it's just a small age range, so in 8 years it will be an entirely new group of people. Fifteen years is nearly two full lengths of that duration, so it's no wonder that AIM/MSN/Yahoo aren't in use by the current 12-20 year olds. It's only very recently that I (a 29 year old) finally phased out AIM in favor of other channels. Once you capture a social group, it's fairly stable, there's just a constant stream of new social groups as each year's new 12-20 year olds start going online.
The 12-20 demographic is mercurial, but 12-20 year olds are not.
Though really my point was this: it isn't the case that you need to continually re-grab the same users, it is the case that there are continually new users to grab.
