I think this graphic in the NYT, based on Pew Research data, is really fascinating. It's looking at numbers of self-identified Democrats and Republicans, broken down by age group and by who was president when that age-group was 20 years old. One of the things to take away from this is that the political climate of your early 20s largely determines your political leanings throughout the rest of your life, on average. So we see that people who were 20 when Truman was in office or when Nixon resigned are very likely to be Democrats now, whereas people who were 20 when Reagan was in office are now likely to be Republicans.
I was 20 in 1997, as Bill Clinton began his second term. So my early twenties stretched from the beginning of that term, through Newt Gingrich's reprimand for ethics violations, the Ken Starr investigation, the shady 2000 election, September 11th, and Bush's first couple years in office. We'll have to see how I pan out later in life.
I can think of many examples who buck the trend of this data, my own father being an interesting example.
Update: Another thing I just noticed that's really interesting is that the gap in years between the most Democratic and least Democratic generations is much larger than the gap between most Republican and least Republican. For Democrats, the lowest numbers are for the age group around 71 whereas the highest numbers are for age group 21. For Republicans, the highest numbers are for age 36 and the lowest are for age 24. For one thing, this shows a precipitous drop in self-identifying Republicans since Reagan left office, a relatively short period of time. Bush II certainly has dismantled conservatism.
I think that Democrats should take heart from this data. If we think of the political power being wielded, at any given point, by people between the ages of 50 and 70 or so (i.e. the age group from which our politicians themselves are usually drawn, and the age group most likely to get out and vote), then we are currently going through a Republican swing, but there will be large Democrat spikes in that age-range in years to come. People who are just over 50 years old right now are very likely to be Democrat. There may be another Republican spike in about 15 years, but after that it will largely favour Democrats. Overwhelmingly so, according to this data.