1.) I believe that if we measured individuals in the age range of 25-45, including dual income, and with 2 kids, it would give us the best measurement of how the middle class is doing. This measurement could probably be acquired through suburban developments, targeting suburban areas outside of big cities. This is all just speculation but I believe it would give a relatively accurate reading. OR, we could just take workers from separate companies in the same industry who perform the same jobs and compare them.
2.) I believe that as of now, as it gets harder to retire, that yes, elderly people should be included, because we cannot say for certain that all elderly people in an age range have chose to retire, a lot of them like their work and continue to do so.
3.) It matters if the composition of the middle class is changing because it can't be accurately compared to the year desired to compare to. If the age of middle class is increasing as years go on, then that can communicate social change, or communicates that the economy is making it harder to even attain middle class itself.