In college and grad school: about 20 million
Housewives: about 20 million
Retired, under 65: about 10 million
In jail: about 2.5 million
That accounts for a lot of them.
That "Housewives" needs to be updated to include "House husbands" who stay at home while the wife works. Quite a few of them, and acceptable to young people nowadays. (Not in my day, but things have changed.)