Obligatory reminder that in 2012, Romney only won the under-65 crowd in MS by 5 points:
Relatively young (i.e. non-ancient) whites in Mississippi are probably no less Democratic than whites in, say, Georgia. They're not that much of an anomaly. You're still looking at a majority-white electorate (anywhere from 50/50 to 55/45) even in the 18-29 grouping, meaning that anywhere from 20-25% of those whites are voting Democratic.
It's pretty obvious when looking at both the exit polls and general demography of the state by age that older blacks do not really vote at all in Mississippi (the legacy of the times in which they were raised). This is the predominant problem with flipping MS, but once they - and their white cohorts that are voting 95% R - pass on, the state becomes much more competitive.
Not necessarily, a lot of liberal young people in Mississippi probably leave the state and that is probably a major factor in why the older-cohorts are so Republican-voting.