I don't think you can extrapolate the voting habits of the 18-29 year old cohort out as they age because these young Democrats probably leave the state at higher rates than younger conservatives.
Mississippi is the worst state in the country for college graduates, while the cost of living here is phenomenally low compared to places even like Metro Atlanta and Nashville there is still a heavy brain drain occurring. I think a lot of the young talent the state loses to its neighbors would be from more Democratically-inclined youth.
I don't believe that was what I was saying: mainly, I'm looking at how the middle-cohorts (30-64) are voting right now and extrapolating that into older age (both the 65+ and younger categories).
With the 18-29 group specifically, I'm operating under two assumptions: 1) that future 18-29 year-olds won't become
more Republican and 2) that future generations of that age group will remain in the state for similar periods of time and in similar numbers to register their voting preferences (i.e. as long as they're in college).
Even if today's youngsters leave in mass numbers, they'll be replaced by future generations of youngsters and I don't necessarily see that cohort leaving at younger and younger ages (and in larger numbers) than today's youth.