Good question, I'm really not sure at this point, but I do plan to not be dependent on anything by that stage.
Of course in many ways drugs can reduce a person's life expectancy, or negatively affect quality of life on physical or mental health, indirectly or directly - although things like LSD dont have any effects on physical health, AFAIK.
So, many drug users wont necessarily make it to a very old age. Probably more likely not to, as a whole. There's probably been studies done on this, but I haven't looked tbh.
Famously, even Keith Richards has given up everything by this point, he's 81 now I believe, and I forget the exact decades but it was something like he gave up heroin in his 50s, drinking in his 60s, and smoking in his 70s. Everything may have been one decade later or earlier, I cant remember where I heard him talking about this in an interview to check.
Peter Perret of The Only Ones after decades of heroin and crack, is now 72 years old, and his health seems to have improved. I assume he's given up his vices by now.
Also John Cooper Clarke has also given up all drugs some time ago, also after decades of use of heroin, and now seems in good health at 76 years old.
I know these are all public figures, but I don't know of any old (and by that I mean pension age) people IRL or on line who continue doing drugs. There was one old guy at a drugs recovery place I used to go to, and he was embarrassing tbh, addicted to drugs and acting up like a child at 70 something years old, wouldnt really be a position I'd aspire to be in by that age. (If I make it that long.)
I think it's a bit like severly obese people. You just dont see any of them make it to old age. Presumably they've either had to lose weight, or they've died early.
I know of an example of this that is very close to home. My Grandmother was a large woman and she was a drinker. Her drinking peaked after my Grandad died. After a few months of really heavy drinking she gave drink up completely. And lost a lot of weight. She was no longer over weight. She lived to be 95 years old, IIRC.