Beginning with those between 45 and 49, women became the majority in every age group.
Of the Oklahomans 65 or older last year, 235,224 were men, and 299,023 were women.
Of those 85 or older, 22,338 were men, and 43,222 were women.
Only 10 states had more men than women in 2012, the Census Bureau report shows.
Besides Oklahoma, four states and the District of Columbia experienced a drop in the median age.
North Dakota, which has attracted thousands of oil-field workers in recent years, had the biggest drop — 0.5 years.
Other states with a decline were Hawaii, Alaska and Kansas.
According to the data, McIntosh County, in eastern Oklahoma, had the highest median age of the state's 77 counties — 48.1.
Payne County, which includes Stillwater and Oklahoma State University, had the lowest, at 27.2
Oklahoma County's median age was 34.2, the same as in 2011.
In the 2010 head count, the median age in Oklahoma County was 34.3.
The average age in Cleveland County last year was 33; in Canadian County, it was 35.8; in Pottawatomie County, it was 37.1; and in Logan County, it was 38.