The relevant question isn't whether most people's earning potential goes up over time. That's pretty well baked into most jobs (older people manage younger people and get paid more than their charges, for example). So yes, you should make more at retirement than at 18.
The relevant question is around mobility between quintiles. Put another way: how likely is a person to retire in a higher quintile than the one in which he/she was born? The data pretty conclusively show that the US is calcifying in this regard. In other words, it's more difficult to move up to a higher bracket now than it was 30 years ago, etc. I don't have a citation handy, but I believe Elizabeth Warren has done some research into this area (as have others).
The relevant question is around mobility between quintiles. Put another way: how likely is a person to retire in a higher quintile than the one in which he/she was born? The data pretty conclusively show that the US is calcifying in this regard. In other words, it's more difficult to move up to a higher bracket now than it was 30 years ago, etc. I don't have a citation handy, but I believe Elizabeth Warren has done some research into this area (as have others).