The point at which you start "feeling old" comes suddenly upon you, out of the blue.
One moment you are young, and the next, you are on the downslope, it seems. And yet, it is not the true point from which you actually start becoming old.
It is merely the point at which you need to snap out of your rut, re-think your life and address your vital needs.
Life has just smacked you on the nose -- wake up!