Is there any scientific evidence that show a human being is capable of cultivating internal energy?
Seriously, I don't know because the concept of QI is pretty complicated. Awhile back, I actually did ask in this forum the question of what's the difference between external and internal strength. I think some Kung Fu expert said that they're basically related -- the internal strength is reflected by the external strength.
Some examples in JY literature supported this theory while others refuted it. For example, YG's ability to wield a heavy object was directly linked to his increase in internal strength while Yideng's immense internal power didn't seem to be indicative of great physical strength.
Because there aren't conclusive evidence as to what internal energy actually is, I can only speculate that women are less capable of cultivating internal energy than men since internal energy is a physical trait like strength, speed and agility. However, based on real world evidence, I think it's safe to say that a female warrior would never be able to train herself to reach the levels of top male warriors.
That's an entirely different topic but I would say that JY is cognizant of the debilitating effects of aging. I haven't read enough GL or other authors to know where they stand on the effects of age on combat ability but JY often mentioned that older people are more susceptible to running out of stamina. There are many examples in JY's stories of older characters who were defeated or even died because their bodies simply got worn out.
Remember that Chinese idiom about the the oil running dry and the light going dim or something to that effect? (I can't type in Chinese, sorry
) Sadly, that saying held true for many old warriors in Wuxia.