Mostly flat from 2010-2021, with a recent uptick to 131 million. The discrepancy is likely due to the boomers aging out of the category, and a smaller generation coming in.
Let me put it another way: the [20, 25) and [25, 30) age cohorts are larger than any cohort aged 50+ that might have recently aged out. So that "prime age" workforce is still growing.
This could be true, but it isn't obviously true (to me). (I dispute a little bit the idea that there are many new workers in the [25, 30) demo.) There are 37M workers 55+, but only 20M in the 16-24 range: https://www.bls.gov/cps/cpsaat18b.htm (2024 numbers)