Contrary to what others are saying, I think there is something to what you're saying, but I don't think it is something specific to female singers.
Many artists have their greatest creative periods earlier, rather than later in their careers. Whether this is related to no longer having the time required, a loss of the drive to create or just that life becomes much easier, well you would have to ask them. All I do know is that stress and pain are great sources of inspiration and there is a lot less of both, when fame and fortune are achieved.