Are there stories of a hero or god fighting off old age?
Unfortunately, the mythic story of Heracles fighting off Geras has not survived. I'm wondering, however, if there are other stories of a hero or god2 fighting off old age, especially in cultures that would be aware of the Heracles mythos.
I realise it will not be possible to say that these stories - if they even exist - contain traces of the Heracles and Geras story, but I thought it would be a fun exercise to collect them and get at least a faint idea of what a Heracles fighting off Geras story might have been.
1 British Museum, 1772,0320.423
2 Heracles is both, so we can't limit this to heroes.
The one story that I found while researching the question, is the story of Thor wrestling Elli1 in the Gylfaginning:
And again he said: Let me see first! Call hither that old woman, Elle, my foster-mother, and let Thor wrestle with her if he wants to. She has thrown to the ground men who have seemed to me no less strong than Thor. Then there came into the hall an old woman. Utgard-Loke bade her take a wrestle with Asa-Thor. The tale is not long. The result of the grapple was, that the more Thor tightened his grasp, the firmer she stood. Then the woman began to bestir herself, and Thor lost his footing. They had some very hard tussles, and before long Thor was brought down on one knee. Then Utgard-Loke stepped forward, bade them cease the wrestling, and added that Thor did not need to challenge anybody else to wrestle with him in his hall, besides it was now getting late. He showed Thor, and his companions to seats, and they spent the night there enjoying the best of hospitality.
Prose Edda/Gylfaginning. (2013, January 29). In Wikisource . Retrieved 11:21, March 15, 2017, from https://en.wikisource.org/w/index.php?title=Prose_Edda/Gylfaginning&oldid=4271175
Tacitus mentions in Germania that Hercules was venerated by the Germanic people. The more likely explanation is that Tacitus meant Thor, and this is an instance of interpretatio romana, as andejons notes in the comments and Semaphore in a related answer.
However, temples dedicated to a syncretic Hercules Magusanus have been found in the Batavian region, which tells us that at least some Germanic people found Hercules interesting enough. The possibility that the Thor and Elli story was partially inspired by a Heracles and Geras story is certainly very slim, but I don't think it can be entirely dismissed.
That said, we can't ignore that Elli is female and that Thor loses the fight. Neither detail fits a Heracles fighting off Geras story: Geras is obviously male, and Heracles would usually win his battles with deities.
1 This story is also notable for being an early record of Glima, the martial arts system of the Vikings.
This is hardly an obscure story; by the standards of norse mythology, this is almost as close to canonical you can get! Furthermore, it should be noted that Utgard-Loke afterwards admits to being utterly terrified since Elle only brought Thor down on one knee. Lastly, "Hercules" being venerated by the germanic peoples is clearly *interpretatio romana*, and in all likelihood Tacitus actually meant "Thor".
In the ancient Eastern historical accounts of great kings, there is a mention of King Yayati who was cursed to become old & invalid. However, he was able to exchange his old age for youth from his youngest son, Puru.
A detailed description of the entire story can be found in Srimad Bhagavatam (spoken & compiled about 5,000 years ago) - Canto 9, Chapter 18 - King Yayati regains his youth.
I found this web link for you which has the full description, verses and translations: http://www.vedabase.com/en/sb/9/18
Very interesting story and hope my contribution helps.
This is a very interesting story, indeed. However, at least the way I read it, it's not really a story that fits the motif of a hero fighting a personification of old age.
It little profits that an idle king,
by this still hearth, among these barren crags,
match'd with an aged wife,
I mete and dole unequal laws unto a savage race,
that hoard, and sleep, and feed, and know not me.
expressing, his sense of disconnection, being last among his generation, friends all dead; his age, via reference to his once legendarily desired wife; his idle (purposeless) existence; and the mundane world he is now hopelessly entangled in.
I cannot rest from travel: I will drink life to the lees:
all times I have enjoy'd greatly, have suffer'd greatly,
both with those that loved me, and alone, on shore,
Referencing his most famous of stories, great not only for his exploits but because of his legendary suffering, being the lone survivor. He speaks of his raging against the dying of the light in his restlessness and thirst for life, and recalls how he
drunk delight of battle with my peers,
far on the ringing plains of windy Troy.
How dull it is to pause, to make an end,
to rust unburnish'd, not to shine in use!
as tho' to breathe were life!
He goes on to talk about handing his kingdom down to his son, of his lost companions from his famous journey, all lost beneath the gloomy seas, protesting that
Death closes all: but something ere the end, some work of noble note, may yet be done, not unbecoming men that strove with Gods.
And, ever defiant, Ulysses concludes:
'T is not too late to seek a newer world.
Push off, and sitting well in order smite
The sounding furrows; for my purpose holds
To sail beyond the sunset, and the baths
Of all the western stars, until I die.
It may be that the gulfs will wash us down:
It may be we shall touch the Happy Isles,
And see the great Achilles, whom we knew.
Tho' much is taken, much abides; and tho'
We are not now that strength which in old days
Moved earth and heaven, that which we are, we are;
One equal temper of heroic hearts,
Made weak by time and fate, but strong in will
To strive, to seek, to find, and not to yield.
Mega props for bringing up Tennyson. We don't see nearly enough references to literature, and I had all but forgotten this poem! Tennyson's Ulysses was taught in my classics program, along with Joyce's (and, really, as much other significant literature as they could cram in. It's a huge part of the point of studying the classics :)