Nate Silver of the politics/statistics website Five Thirty Eight has some graphs up, based on recent Gallup surveying. The most interesting graph, to me, is the one that plots current (early 2009) party identification (Democratic lead over Republican) on the y-axis, as a function of respondent's current age, on the x-axis. To bring out the key message, however, the graph adds color shading and name labels to show who was president when the respondent was 18 (e.g., someone who is 42 years old today would have been 18 in 1985, when Reagan was president).
The Democrats have greater party ID than the GOP in all age groups today, but the margin varies quite a bit. Among respondents whose 18th birthday coincided with George W. Bush's presidency, the Democratic party ID edge is huge, upwards of 18%. On the other hand, those who were 18 at some point during the generally successful and popular presidency of Ronald Reagan (until scandal hit in his sixth year in office) are roughly equally distributed in their support for the two parties (which is as good as things get for the GOP today).
These findings suggest that the late teens and early adulthood may be formative times for lifelong political affiliations. The graph in question is the second one down you'll see after clicking here.