To the OP: great thread. We all think about these issues as we age.
I find as I get older my tires get a bit fatter (I tend to run 32c tires now), my handlebars are a bit higher (I run them level with my saddle and I like randonneur bars), and my gearing is a bit lower (I like triples and compact doubles, racing cranks not so much) but my fun quotient has not gone down. I just do things a bit different. I always sign up for at least one really hard ride a year (a metric century with 5,000 ft of climbing). If I finish that ride and I'm too beat to drink a beer, I figure I've done the ride all wrong,
.