Certified GKH Gardening Expert
There are many issues and diseases that cause these symptoms. Generally, as Cherry trees age they will start to lose vigor. They may even start the process of dying after about 15 years, or so. Between 15 and 20 years will be an expected lifespan for a flowering cherry, though they can live longer.
If you believe that the trees are reaching their life's end, then this is likely the cause- Natural senescence.
Here are some articles that will offer some more insight: