Although the peak onset of depression symptoms is between the ages of 15-24, depression can hit at any point in the lifespan and, for many, can be a lifelong struggle. People who suffer from depression as young people are also more likely to experience depression during adulthood. We also know that depression is common among the elderly and is often confused with the symptoms of other problems that arise during the later years.
More information about depression at different life stages:
Children and adolescents
College-aged adults
Older adults