How does a child develop aortic stenosis at such an early age, isn't this a disease for the elderly?

Not Only Elderly. You are correct in that as is common in the elderly. But children are not little adults, and the process is different. In children with, the aortic valve itself usually did not develop normally. Or, they contracted a disease like rheumatic fever which affects heart valves (quite rare in the west). Please discuss your specific issues with your cardiologist.
Not necessarily. Aortic stenosis can result from a malformation of the valve in a fetus. It is often detected by its high pitched murmur in the first weeks after birth & can be helped by doing a catheter based treatment soon after it is found. The as of an older adult is quite a different issue.
Heart formation. Aortic stenosis in a child is due to abnormal heart development. The normal valve has 3 leaflets. The individual valve leaflets may be fused together or malformed.