Growing older and more mature?
Closing one chapter and starting another one?
Realizing you are no longer supposed to be a child?
Discovering, at the same time, that all you want is to remain a child?
Is it more white hair?
Or less energy?
Is it leaving most of your emotional reactions and behaving in a more logical manner?
Or making peace with yourself?
Is it just a limit, or an ordinary number at all?
The end of anticipation and the beginning of acceptance, probably... That defines it much better...