The rut

The vehicle is something to look forward to at the point in your life when you have run out of things to look forward to. When you are falling gracefully toward adulthood this isn't the sort of thing you think about. There is still your first kiss, losing your virginity, your first job, and all sorts of other things to look forward to. Then at some point after I started looking back to find the point at which my life's derivative reached zero. Is this a mountain or a range? How long can mountain climbing maintain interest?