These are very understandable fears to have. I fear one of them very much. Aging to me is far more scary than death.
One of my worst fears (which ties directly into my worst fear) is becoming powerless, impotent, and senile. I never want to be helpless. Helplessness strikes harder than any physical wound.
Loneliness I don't fear very much, as I am alone often. I prefer solitude most of the time, and I feel that I am one of the few people that can actually claim that. Many say they like being alone but if you watch people it is amazing the lengths people will go to, to have interaction with another person. Every now and then I find myself doing something so that I can talk to someone; like I will go to a store when I am bored not to buy things, but in the hope that I can meet someone. Obviously for most people these aren't conscious decisions (going to the store is) but we work so hard to feel connected to something larger than ourselves. Whenever I catch myself doing this I always feel weak and stupid.
My absolute worst fear (I fear it even more than I fear I sharks
) is to not accomplish what I am able to. I fear not only not reaching my potential but not doing anything that makes my life worthwhile. I want to contribute to humanity, not just exist.