Inventiveness, ability to mask your intentions, good distance handling, technical knowledge, calmness, controlled agressiveness, good technique and much more are not so much related to age or physical capacity, but rather dedication and experience.
An experienced 50 year old fighter certainly is a match for a less experienced younger fencer. But, given that both have equal experience and stamina, age can be a hinder, since joints start to protest etc.
It also depends on how you look at it. In a real self-defense situation with weapon in hand it might be over with one hit and therefore stamina can be less relevant. In a sportive context, with counting hits over a certain period of time, this might be different.
I can tell you this though, my situational awareness and reflexes have improved a lot due to longsword training. It is apparent in ridiculous situations like always catching things that fall from shelves and similar. There is a lot more room for improving ourselves than we tend to think, even in less apparent aspects.
Me, I am 42 and I have had knee surgery and I do well enough against fencers half my age, I think. Still, when they are my age they will be wicked if they keep it up. I look forward to watching amazing swordfights in the World Championships on TV, when I am old and retire...
