It never has. The labor force participation rate for 25-54 year-olds is a better metric for such things.[1] Last time it was this high was 1990s through 2002. (Before that, it was never this high.)
The shape of that graph is roughly equivalent to the shape of labor force participation for women [1]. I don’t think that detracts from your point in regards to the last 20-30 years, but in regards to “before that it was never this high” I think it’s evident that the societal shift of women joining the workforce is the reason, not an improvement in the economy.
[1] https://fred.stlouisfed.org/series/LNS11300002