I need help writing an op-ed that supports one proposed policy related to the public health issue of depression in older adults.
If your instructor is assigning you an "op-ed" piece, then you need to write in an engaging style for a popular audience. This means that although you do need to do research and find authoritative sources of information, you still need to write in a non-technical style and clearly show the relevance of your claims and suggestions to the putative readership of your piece. Given the aging of the newspaper readership, with almost half of people who read newspapers daily being over 65, you should write in a way that assumes your readers or their spouses or friends may be directly affected by this issue.
There are many public policies you could support, depending on where you are located and your actual audience. For example, if you are in the United States in one of the states where a Republican governor or legislature has turned down federal money for expanding Medicare, you might advocate for changing that policy to enable greater support for mental health care for seniors.
Another possibility would be advocating more public funding to support training doctors specializing in mental health care for seniors, or other aspects of geriatric medicine, possibly in the form of forgiveness of school loans for doctors specializing in those areas.