a) The GI bill was an exception to the rule. I sincerely doubt that people earning BAs in their 40s are as common nowadays as you claim they are. At least not more than common than in Germany. (Yes, it happens.)
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.
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.
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.