Is this fair? As I get older, 41 now, I'm starting to see why older people jump less, and less high. The effort of jumping can be painful. And pain is such a good trainer, if it hurts we do it less, and if we do it less we do it not as well.
If you fight in the ring, you need a lot of stamina because staged fights are artifically long for enternainments sake. And the longer you do something in a short period of time, the slower it tends to get. So for ring fighting, you need to train speed.
But in a real fight the length of time is likely to be short. In a real fight the pain an older person might have from moving fast will likely be covered up by the hormone cocktail. It will only be felt afterwords, perhaps even days later. So training speed is not necessary.
Instead of training speed, internal martial artists train smoothness. And we train integrity. Because, although an old man can pull his hand out of the fire as fast as a young man, for the old man the very act of pulling out at high speed is likely to tear a muscle in the elbow which takes months to heal. While the young man will likely say "ouch," and forget about it.