You're not old. I returned to the fold after a 28 year absence. I'm 60, do a minimum of 15 miles a day and am having a wonderful time.

If you're happy with your bike, keep it. One of the big strengths of the Fuji line is that they never made a bad bike. Anything with their nameplate on it was (and is, for that matter) quality for that specific price range. I'm always happy to point a newbie in the direction of a Fuji, whether he's looking at vintage or new.

As to the Sagres, looking at the specifications from the catalog page, everything that it comes with is nicely functional even thirty years later. Assuming the Accushift is still indexing properly, any changes you make on that bike should be limited to personal comfort. I'm riding equivalents to that bike on a daily basis now, and find them just as good as the three modern bikes I've got in the stable.