There are several different 'slow-down' elements.
1. You have less time to train
2. You are tired of training so much and cut back
3. You get injured more for two reasons: a) older; and b) accumulated overuse injuries etc.
4. Your body does not let you train as much
5. The body cannot perform as well
The general notion of limit is 5 but all of these can be factors, and some of them become non-factors (e.g., 1 and 2) if you make the decisions to train.
When the physical elements (3-5) kick in differ substantially, especially the aggregate of the three. One way you see this is that the distribution curve gets much thinner at the top, although the bump at each 'age-level' (e.g., 40) shows you more what the true limitations are as people make different choices on 1 and 2, and by going hard for a couple of years and easier for a couple (e.g., 40, 41, then easier 43-44) you give your body a break.