First, the BLS provides seasonally adjusted data for the "Prime" working age group (25 to 54), but only NSA data for the 55 to 64 cohort. So, this makes tracking a larger Prime+ group more difficult.
Second, the reason we use the 25 to 54 group is that the participation rate is fairly steady across the entire age spectrum.
If we added 55 to 59 to the prime group, calling it prime+, we'd have to age adjust to track the participation rate over time.
Although the participation rate is increasing over time, it is still below the prime levels. When a large cohort is moving through that age group, that would push down the prime+ participation rate, and when a small cohort is passing through that would push up the prime+ participation rate.
This graph shows that older people are participating more over the last 40 years, and younger people are participating less. This is probably because people are healthier (older participation rate), and younger people are staying in school. Both positives!