I think it all depends on events that happen in each individuals life, and the choices they make in those situations. For instance, many people who end up healthy, wealthy and wise are very hard-working, watch what they eat, etc. This takes a lot of determination and focus that others are simply not willing to take.
Others may be influenced by harsh realities that make them grow increasingly bitter until they do something that ends them up in jail.
With the happier/ more depressed with age... I believe it is all down to a person's choice. If they focus on negative things their whole lives, they will be miserable. If they find the good in many things, they will be happy. Thats why you will find people who are both healthy and wise that are miserable and low-income families that are happy. It doesn't make sense to an outsider looking in, but it is the choices they made.