The two plots together suggest that the Plains are losing population in all age groups are are not getting older.
The surprise for me is FL. Not only is the youth population rising, but the share of the youth population is rising, too. Orlando, Tampa Bay, the Panhandle (except Pensacola), and Metro Miami all saw an increase in the youth share.
In 1970, I imagine persons aged 25-34 would have been more likely called parents, rather than youth. These would be persons born during the Depression and WWII.
With lower birth rates due to the Depression and WWII, these would represent an atypically low share of the population pyramid.
1970 USA Population PyramidIn rural areas this number would have been further depressed due to their parents being forced off their family farms. The baby boom further reduced their share of the population. Between 1950 and 1970, the male share of the population 10-14 in 1950, decreased from 3.7% to 2.9% in 1970 when they were 30-34.
It would have been excess baby boomers who would have left when they reached adulthood.
25-34 year old in 2010, were born between 1965 and 1975, post baby boom. Since then the population pyramid has flattened considerably, with aging (and dying) baby boomers representing only a slight broadening at the shoulders, compared to the broad hips they had once represented.
The share of 30-34 year old in 2010 is 3.3%, only slightly less than the 3.6% they represented as 10-14 year old in 1990. Further, they represent a considerably larger share of the overall population than those born 40 years earlier, despite being born post baby boom.
2010 USA Population PyramidA more stable population structure, along with stabilization of the overall population translates into a larger share of the population in rural areas (eg a county that might have dropped from 10,000 to 5,000 between 1910 and 1970, may have only dropped to 4,000 by 2010.
In Florida, you are seeing a decrease in the share of 25-34 in more modern retirement areas, such as The Villages and Fort Myers. In areas that have been retirement havens for a longer time, you see an emergence of the population that moved to provide services, etc. and their progeny, and the retirement population stabilizing, as the older retirees dying off or moving back North to be with family, and some newer replacements. Retirees typically seek a low cost area where they can live on SS and whatever savings. They may be priced out of places like Miami.