I think the second explanation is correct, especially since your life up to the present doesn't have a definite beginning point in your memory. Even if there is a first thing that you remember, you also know that that was not really the beginning of your life. So your life as you remember it is basically indefinite, but it is still objectively a finite quantity of time. And since you don't have any particular objective measure of time, the only way you can measure a month or a year passing now is to compare them with your past experience of time. This gives you a fairly precise measure of how much time should be appearing to speed up. For example, the time from age 10 to age 20 should pass about as quickly as the time from age 20 to age 40. In my experience this seems about right to me.