system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] What happens to old Imperial Guardsmen?

I am an imperial guardsman from a small backwater planet, and have served the Emperor with honor these past 60 years. I was a Sergeant; lost my right leg to an Ork and my left hand to pirate scum. I thank the Emperor every night, for many I served with received fates far worse.

With the dusk of my life upon me and snow in my hair, can I ask what the the Emperor has planned for me next? I desire greatly to see my fair daughters, now in the middle of their life. More than this, will this old soldier will receive a stipend? Farming at my age would be a trial indeed.

Thank you, and may the Emperor smile upon you!
gpt
You've mentioned that your world isn't famous.  You're not from Krieg or some other outrageously hardcore world where you fight till you die.  And contrary to what you might think, you're not the only person who's become old in the Guard.

Guard regiments have several different policies for retirement.  The most typical is that you serve for a fixed term: 15, 20, 30 years, that sort of thing.  Obviously if your regiment can't be replaced quickly you may spend a few more years than you expected in battle, but overall you're going to get to retire if you somehow live to grow old.  Even the Cadians have something like this, since new Cadians have to come from somebody.  Yes, some battles go on for generations, but those aren't the sort of battles that people normally get to your stage in.  Regiments are also often disbanded for fresh ones, and while the best of them are merged to other regiments, the soldiers that aren't needed any more get their own arrangements.

However, retirement isn't the same as being shipped back to your home planet.  This is a backwater, you say, presumably not near any of the major destinations of the galaxy.  There can't be too many regiments from a place like that, and it can hardly have that much demand for fresh soldiers.  You might well be able to settle on a new planet, freshly conquered or re-settled, near where your last military campaign was.   

From there, if you want to head home, you'll have to hitch a ride.  Any pension will have been the land you were given to re-settle, so you'll probably want to sell that.  Traders will ship you, for a price, to various places, and there's also room for an old member of the guard in various job positions: you made sergeant, so you might be able to do consultancy work for a local Planetary Defence Force, or join up with a Rogue Trader or a Mercenary Company.  It might take a long time, but if you stick with it for long enough, eventually you can get home.  So long as you're allowed to retire.