Aortic stenosis is most commonly caused by age-related progressive calcific aortic valve disease (CAVD) (>50% of cases), with a mean age of 65 to 70 years. CAVD is the build-up of calcium on the cusps of the valve, and this calcification causes hardening and stenosis of the valve.[22] Another major cause of aortic stenosis is the calcification of a congenital bicuspid aortic valve or, more rarely, a congenital unicuspid aortic valve. Those with unicuspid aortic valves typically need intervention when very young, often as a newborn. While those with congenital bicuspid aortic valve make up 30-40% of those presenting during adulthood[23] and typically presenting earlier (ages 40+ to 50+) than those with tricuspid aortic valves (65+).[7]

Reference
Wiki: Calcific aortic valve disease