Read this article : https://www.webmd.com/depression/guide/depression-elderly#1It. It helps you to understand the correlation between older adults being more susceptible to depression.

Often at the older stages of your life, many people begin to question themselves and their accomplishments. Many people begin to questions whether they've made the most of their time on Earth, what they could have done, things they shouldn't have said, promises they never kept, etc. These thoughts, often accompanied by a sense of worthlessness and failure, take a toll on the mind and often lead to depression and other mental issues.

From a medical perspective, for adults who experience depression for the first time when they grow older, the depression may be related to changes in the brain and body. Ischemia, for example, restricts blood flow to all organs, including the brain. This condition, called vascular depression, may cause the individual to become depressed even if they have no family history of depression.

I think some of the main reasons for elderly depression include a sense of failure and isolation. When the elderly retire and their health begins to fail, they are often forced to stay at home and watch as the rest of the world speeds ahead in development, seemingly leaving them behind. From this point of view, it may appear as if their own accomplishments had hardly been meaningful at all in the grand scheme of things. In addition, as the elderly's children have children who are engaged in their own busy lives, the elderly may only rarely get to see their family; this situation is also often exacerbated by the death of a partner. When these and more factors come together, many elderly people fall into depression.

I think that it is less so that the elderly are more "susceptible" to depression, but rather many elderly individuals feel that they have come to the end of the road and have nothing else to look forward to in their life. This can cause feelings of deep depression. Death seems much closer and there is less to do as other health issues arise and individuals must withdraw from many social aspects of life. The elderly may also be dealing with more loss of loved ones or friends, causing more sad feelings.

Elderly people may also experience more traumatic events than the normal person — as their friends and family age with them, death may be more commonly seen in the life of an elderly person rather than in the life of a teenager.

What's important to note, however, is that depression is not a normal part of aging. In fact, most older adults feel satisfied with their lives compared to the average person. Depression is usually a complication that comes with long-term physical illness, isolation, or chronic pain.

I think elderly people can be especially susceptible to depression for a variety of reasons; most notably, I think that the declining quality of life as one ages can be incredibly discouraging. As we get older, and our health begins to deteriorate, it’s easy to get into the mentality that, because we can’t do all of the things we used to be able to, we can no longer find things to enjoy. Furthermore, the creation and increasing reliance on techology lead the elderly, who grew up without any modern technology, to feel isolated.