Otherwise healthy, I had a rapid decline in vision when I was about 16 or so, but it's been unchanged since that time. Can anyone think of a reason why that might have happened?

In what way did it decline? Do you just mean that you had to wear glasses when you reached that age? If so, that's typically how it happens. When you are young, you have the strength in your eye muscles to deal with, particularly long sightedness but when you reach late teens/early twenties you lose that ability and optical correction becomes necessary.

It seems fairly common. My son nearly failed his visual driver's test because of his sight and we had no idea it was bad.