AFAICS, "over the hill" will come when my body no longer can deliver what my mind wholly desires it to do, despite whatever efforts I make to coax it, strengthen it, heal it, or rest it into the proper shape.

Therefore, a pro athlete may be over the hill at 35, because he (or she) expects nothing less than exceptionally superior performance every time. Conversely, if I keep my expectations somewhat in line with my physical capabilities, I may never be over the hill.