The question of "When?" is always a very personal one. I was initially diagnosed at age 52, with moderate-to-severe AS. My only symptom was gradually diminishing exercise tolerance. I had NO other symptoms, being a daily runner/jogger for over 30 years, in addition to being a confirmed gym rat (weights, cardio, etc.) and just "living the dream." My first cardio and I used to play a game we called "Stump the Fellow." In this game, he would direct new cardiology fellows to interview me, examine me, then diagnose me. They almost always blew it, becoming very concerned when they heard what were my "normal" activities, then heard my mammoth, late-peaking murmur. I was more active than most men half my age, yet had this very ominous murmur. The message they were given was to never jump to conclusions, but to evaluate the entire patient, not just one test or another.
Fast forward about 11 years. I was still pre-surgery, and had transitioned to cardio #2 (#1 moved away). We were having the usual discussion of "How will I know when?" Cardio #2, wise beyond his years, simply told me "I have never had to tell an AS patient when they should have surgery. They always tell me when they are ready." He was completely right in my case. About a year later, I told him "I'm really tired of being tired. Let's do it." The rest is history.