You are free to kill yourself when you feel like you had enough. Just because you don't want to have the body of a thirty year old forever, that doesn't mean that you should subject others to the incredible suffering that old age brings.
We are not free to end our own life when you feel you had enough. There's laws for that. But yes, I'll reflect on my options whether or not I'll have that chemo when I'm in my sixties/seventies.
On the other hand, they could do that if you don't. There are a few scifi novels where the concept of forced retirement (in form of euthanasia) is explored.
