This could also represent a compression of income-earning years. For instance, if more people spend their twenties earning little, but then their income jumps u

This could also represent a compression of income-earning years. For instance, if more people spend their twenties earning little, but then their income jumps up a lot by the time they are 40, they may look middle-income for only a few years. I'm not saying this is what's happening, but based on the numbers and definitions, it's possible. In general, this kind of snapshot definition trying to capture the middle class is misleading. You should follow individuals over their life to see whether they are thriving or struggling. This is not even a class mobility issue. It's more of a life cycle issue.