Unfortunately, your confusion is endemic of the problem with government statistics using disposable income as an indicator of prosperity, and is even being quoted by others in this thread. Indeed, you likely googled "historical disposable income" to back up your statement - which is exactly the problem with using the incorrect term.
I wont make the comparison to 100 years ago (as you did) because that was a period of time including 2 world wars and the great depression., But adjusted for inflation, people generally have LESS discretionary income now than they did in the 60s. Again, this is hard to corroborate with official poverty statistics due to changes in the cost of living (and conversely, the provision of benefits such as food stamps), which are frequently petitioned to be amended.
Over 12% of americans have such low discretionary income they have to rely on food banks - a figure comparable to the 1970s.
10x more people in the UK are accessing emergency food from food banks than they did 10 years ago.
It is a terrible state of affairs, to be honest. Please dont use disposable income :)
