When does aerobic capacity decline?

As soon as you stop using it.

The human body is an adaptive machine. If you are physically active, your body will adapt and improve it's ability to perform those physical activities. If you are physically inactive, you WILL regress and become less physically fit.

True, there is age-related decline as you enter your senior years, but the biggest determinant of aerobic capacity (or any other marker of physical fitness) is how much or little you challenge your body to perform.

Use it or lose it.