This is controversial to most people, but I don't plan to continue living when I get too old to take care of myself or I'm in pain all the time. I've always known that I'll die of suicide. I'll choose how and when I'll go, unless I die in an accident or something before then. I think it's extremely important that people are able to choose how and when they want to die. Obviously you don't want mentally ill people or people who are enduring temporary problems killing themselves, but if an individual is of sound mind and doesn't want to be here anymore I believe they should have the right to choose how they're going to end their life. It's a very personal choice and something that everyone deserves to make.
Now before anyone freaks out here, no, I'm not suicidal and I have no desire to die right now. I plan to be here at least another 30+ years, but I'm not going to spend my last years/months/weeks/days in pain all alone in some facility or hospital. I'd rather just walk off into the woods and die of exposure/dehydration, though my suicide plan is hopefully going to be quicker and less painful than that. If all else fails though, that's my last resort.
Maybe I'll change my mind when I get older, but I don't think I will. I've known many things about the way I was going to live my life even when I was very young. I won't pretend that my life has ended up EXACTLY like I'd envisioned it, but it's pretty darned close.