There are a variety of circumstances leading to AVR and sometimes surprises found only during surgery...
I was 46 in March 2006 when I suddenly developed significant symptoms. I had no idea what the cause was. After a few weeks of ignoring the symptoms, my wife told me to make an appointment to find out what was going on. I went to my primary care physician who diagnosed a significant murmur. Then had an echo done, met with cardiologist, and was told I had a BAV with significant aortic regurgitation. He (and several other people) said I probably had been feeling bad for a long time. I insisted, no that was not the case. Symptoms literally started upon waking up one day in the middle of March.
I asked how long I could go without surgery before it would permanently damage my heart. He could not predict but estimated I would not live to see the end of decade if left untreated. Three weeks after diagnosis, I had AVR. The first time I saw the surgeon and cardiologist after surgery, they both told me I really needed to get this done.
It was a rare situation.
There was a cord attached to the middle of the defective leaflet. The other end had apparently been connected somewhere inside my heart. The heart side of the cord ruptured causing the valve to prolapse and the sudden onset of symptoms.
So in summary, At age 46, my valve was replaced 6 weeks from onset of symptoms and 3 weeks from diagnosis.