For most people, the most fearsome thing about growing old (other than the fact that it ends in death) is the fact that their physical and mental capacities will decline to some greater or lesser extent. The loss of abilities that they take for granted in their younger years is a major fear.
There are physical aspects to this fear. We are a youth-obsessed culture and people do not like the idea of losing their looks. This is one reason why plastic surgery is so popular in our culture. The number of advertisements for things like Viagra on television and in magazines shows us also that the loss of sexual ability is a worry for many men. In addition, people worry about other things like losing their independence as they lose the eyesight and reaction times needed for driving.
There are also fears of loss of mental capacity. For many people, this is even more fearsome than the loss of physical ability. Alzheimer’s disease, in particular, is extremely frightening to many. The idea of becoming completely unaware of one’s past is one of the most horrible things for many people to contemplate.
Thus, people fear both the physical and the mental decline that can be associated with aging.
A person's biggest fear about growing old would be death in my opinion. I think it would be a big fear because they will have to leave their loved ones behind. When one ages, they know that death will be near. Death is very scary and that why I think it is a thing to fear when growing old.