I had the same same thing with the much less erudite Stranger in a Strange Land. Re-read it 20 years later and was disappointed. On the positive side, I should consider how much I've moved forward in that time :) or sideways :-S
The nice thing about Heinlein is he has a book for all stages. As I've gotten older I really prefer his "Time Enough for Love" novel, which is about looking back on a long life.