It seems that the scientific community agrees that the development of new neurons stops between adolescence and early adulthood. ("Does the Adult Brain Really Grow New Neurons?" https://www.scientificamerican.com/article/does-the-adult-brain-really-grow-new-neurons/) Although the exact age might still be disputed by experts in the matter a consensus exists that they definitely stop developing new ones at some point. After this point in time the brain will an ever-decreasing number of neurons every day. At the time of a person's death they will have at least 1 neuron in their brains, which implies that there was at least one neuron with a lifespan of several decades, possibly the same amount of years as the person.
This brings the question, what makes some neurons die at a relatively "young age" while other neurons last "forever"? Is it related to the function (or "type") of said neuron?