DC Econ: How Is the Middle Class Doing?

What group should we measure in order to discover how the middle class is doing in America? We should measure the middle class in the 70s to discover how the middle class is doing currently.
Does it make sense to exclude the elderly when trying to measure the progress of the middle class over time? It makes sense to exclude the elderly as they rely on government benefits because they are retired or work very little.
Why does it matter if the composition of the middle class is changing when trying to assess the performance of the economy? The composition of the middle class is changing and this matters when trying to asses the performance of the economy because it shows how we have more options and different benefits than people in the 1970s, making assessments difficult.