To the wings above water, I am there in an instant;
the shell upon sand may not see me at all.
Once, men honored me as the bringer of wisdom,
but now fear me as the stealer of their strength.
Men shudder at my approach,
but if they knew they would not see me,
they would cry out in anguish.
I have a foe, against whom I seek to protect mankind.
But he is the stronger; I cannot guard all from him,
and even those I protect will fall to him.
Who is my foe? And who am I?
It sounds to me as if
you are old age, and your foe is death.
"To the wings above water, I am there in an instant; the shell upon sand may not see me at all."
The mayfly, an aquatic winged insect ("wings above water") is proverbially short-lived. I'm not sure what's up with the shell, though. -- Aha, aznbanana points out in comments that some crustaceans allegedly don't age.
"Once, men honored me as the bringer of wisdom, but now fear me as the stealer of their strength."
Traditionally the aged are revered as wise elders, but in present-day Western society that happens rather less (mumble literate versus word-of-mouth transmission mumble) and old age is thought of mostly as bringing physical and mental weakness.
"Men shudder at my approach, but if they knew they would not see me, they would cry out in anguish."
As I am fond of saying, getting old is unpleasant but it's much better than the alternative.
"I have a foe ..."
This I confess is not perfectly clear to me; it seems like it refers to death, but I'm not sure why death is particularly the foe of old age rather than, say, of life more generally. (Perhaps just because the old are particularly likely to die.) Anyway, if I'm right about the foe the main point here is that death claims everyone in the end, even those who make it alive as far as old age.
I am
old age
To the wings above water, I am there in an instant;
Not sure about the “above water” part, but flies have very short lives, and so they reach old age very quickly.
the shell upon sand may not see me at all.
A shell is not alive, so it doesn’t age, in the conventional sense of the word.
Or, a shell on a beach may have been shed by a young animal (as it grows), so the discarded shell will not be around to see its donor get old.
Once, men honored me as the bringer of wisdom,
Not too long ago, old people were revered and considered to be wise, based on their experience (and the fact that they managed to survive to an old age).
but now fear me as the stealer of their strength.
Culture is now more youth-oriented; people are afraid that they will become incapacitated when they get old (especially as advances in health care allow people to live to very old ages).
Men shudder at my approach,
Same as the above: people fear the onset of old age.
but if they knew they would not see me, they would cry out in anguish.
As with any hazard, the threat of being caught unawares makes it even scarier.
I have a foe, against whom I seek to protect mankind.
The conventional wisdom is that you have two possibilities: grow old or die. (Dorian Gray is fiction.) People who are old are alive; in that sense, old age protects people (and other life forms) from death.
But he is the stronger; I cannot guard all from him, and even those I protect will fall to him.
Everybody dies eventually.
$\color{black}{\text{I am }}\text{old age }\color{black}{\text{ and my foe is }}\text{death}$
