At least based on some of the things he's written in some of the anthologies, it seems like a lot of that disillusionment was not just because of age, but rather because of his battles with the publishers and what not that were pushing him to make changes that he felt would compromise the integrity of the strip. A lot of the comics include subtle jabs about corporate greed, artistic integrity, etc. because he was actively fighting with the corporations that distributed his strip over such matters...

Still, 100% agreed that he stopped at the right time, both because of the creeping cynicism, but also simply because he was running out of fresh ideas...