Why does it fail? They fail to consider that the young voters in 2008 are likely to have a higher rate of voting in this cycle. The same group of young voters are not the same as in 2008. They are moving out of the young voter category. A big difference is that a voter that starts voting when they are young tend to continue voting regularly.
What happens on the other end of the scale? They die off. I like to tell people in my county that Republicans die off faster. And they do. So the key is to get the young ones to understand Democratic beliefs and vote Democratic.