Not sure what you mean by this considering how states like North Carolina and Georgia have millennial voters that are more democratic than Virginias millennial. Virginian millennials seem to have similar numbers that democrats get with millennials overall.
It's not necessarily about how deeply Democratic they vote, it's about whether they keep voting that way the more they age. In Virginia, there seems to be little slippage, as the 30-39 year olds in 2012 helped make the 40-49 bloc majority Democratic in 2016 as 4 years' worth of those voters shifted into the 40-49 category. If you applied the same to North Carolina, the 30-39 group should be a lot more Democratic than it was in 2016, but it isn't. I don't think population migration is enough to explain this in either state.
Uhh, the exit polls say that Hillary improved amongst the 40 - 49 by one percentage point compared to obamas percentage in 2012 in Virginia, which is completely meaningless and doesn't really tell us anything.