266 Million Americans are adults. 74 million are not yet adults. Some percentage of the adults are retired myself included, some are medically disabled, some are homeless. About 25% of adults (or 33% of all people) are on one or more welfare programs. Some people are career criminals and only the IRS count that employment and revenue.
In terms of people capable of work, that are willing and able to work and want to work that number is indeed 4.3%. That is how unemployment is calculated, not the number of adults that are not currently in a job. Anything outside of that 4.3% are not employable either due to circumstances beyond their control or by choice and that is a much bigger topic that I will defer to someone else to comment on.
Calculating a percentage that is not employable would be rather challenging I think. Whatever that number is I would expect it to climb as more jobs/careers are automated and deprecated, as increasing numbers of younger people are illiterate and probably quite a few other factors.