Students are more transient generally. Once you get a job, you tend to stay in more or less the same place.
But no, it's not an issue of “maturity”, that all of a sudden you get married, buy a house, and you're going to be more involved. Socio-economic realities do have an impact on civic engagement; however, it's more about your first voting experience. If you don't have a good first voting experience when you're 18, or soon after, you are statistically less likely to be involved throughout your entire life.
We're talking about a generational issue here. In 2006, give or take 36% of young people voted. If we look 20 years down the road, that's what we're looking at in terms of the number of citizens who will likely be participating in elections. It's generational.