Pew Research offers a profile of those who supported Howard Dean:
But Dean activists are far wealthier, better educated, more secular and much less ethnically diverse than other Democrats. A disproportionate number of Dean activists are white, well-educated Baby Boomers fully a third are college graduates between the ages of 45 and 64, compared with just 9% of Democrats in the general public. But the image of younger Deaniacs as political newcomers has been borne out. For more than four-in-ten (42%) Dean activists and two-thirds of those under age 30 the Dean campaign represented their first foray into active presidential politics. And among those who were political veterans, a sizable number (36%) said they were more engaged this time than in previous campaigns.
And they "constitute an engaged group of citizens who intend to remain active in the Democratic Party and exert significant influence over its future direction."