October 2009 US statisics:
College students over the age of 35 = 1.18 million full time + 2.01 million part time.
[http://www.census.gov/population/socdemo/school/cps2009/tab0...]
To put it in perspective approximately 1.00 million students enter German universities each year. Considering the three year structure of German University education - a crude approximation would be that there are roughly as many US college students over 35 as there are German university students.
http://en.wikipedia.org/wiki/Education_in_Germany#Student_Po...
b) Changing majors four times would indeed be frowned upon. But that doesn't mean it's impossible or even actively discouraged. Moreover, I don't believe that such radical and seemingly aimless changes are perfectly acceptable (in a social sense) in the US.