Our laws recognize a number of legal ages for various activities. You need to be 16-years old to get a driver's license, 18-years old to enlist in the armed forces, and 21-years old to drink. One can argue that the various legal ages should be adjusted up or down, but everyone generally agrees that there needs to be minimum legal ages for some types of activities.
I think we need to extend that concept. Once you reach the age of 80, a new set of legal rights should kick in. Specifically, you should be able to imbibe any drug you want, and you should have the right to doctor-assisted euthanasia.
A typical 80-year old might need a little chemical boost to make life tolerable. There isn't much chance an 80-year old will join a street gang or shorten his lifespan by much. Perhaps the law could require a full-time nurse or family caretaker to be around if the oldster wants to drop acid. I could see some restrictions on the activity, but it seems cruel to force grandpa to have a crappy last few years when science provides options.
Likewise, euthanasia should be legal after the age of 80. The cost of keeping people alive in the last months of their lives is a huge part of overall healthcare costs, and a big deal to the economy. I say if you're 80-years old, and you want to spare yourself, and society, from a painful and expensive last act, that should be your legal right.