What is this "ageism" business all about ?

Over the years most of us have been told that we are either too young, or too old for certain professions or pastimes. Who is it that determines how old or young we should when we decide to apply, because we have the ability and skill, that we are one day too old to enter a competition, or audition ?
Surely it is known that people develop at different rates ? Of course, one wouldn't expect to win the marathon at eighty years due to the natural degeneration of the physique and energy levels, but in the case of opera singers, the age at which the voice and body mature can vary by ten years - so why not have open auditions with an upper limit of say, forty years ? That is the age at which most opera singers are at their best in my experience. Too many start out in their twenties and are vocally dead by thirty ....!!
In the acting profession it is often claimed that it takes a forty year old actress to play Cleopatra at the age of fifteen - tricky, but possible. So many performing artist are "ageless".....and it is amazing what good lighting, makeup, etc., can do .....