Hi Martin! You make some fair points.
On age categories: Ideally, perhaps, people would be mixed with other people of similar strength or ability, regardless of their age. Separating by age may just be easier administratively than assessing people by height, weight, ability, etc. These days, however, separating by age is problematic, as you get a lot of massively obese kids whose weight could make it very dangerous for them to play certain sports against other children their own age who are much lighter than they are.
On your second point: Of course it could still be entertaining to watch a sporting event that discriminates on grounds of race or gender. Equally, however, it can still be entertaining to watch a sporting event in a non-discriminatory lower division, so I don’t think following my suggestions would mean we would necessarily be missing out on the entertainment factor overall.
Finally, I find it interesting that you write: “To not have a category for females would be sexist if you allow categories of other classes of contestants.”
I don’t think ‘sexist’ is the right word. It is hardly reasonable to say it would be ‘sexist’ to refuse to discriminate on grounds of gender. Perhaps ‘unfair’ would be the word you are looking for. But that’s basically just the sort of unfairness I am highlighting: It seems unfair when one group of disadvantaged people (women) get prestigious and financially-rewarding special events, when other people who may be equally disadvantaged don’t get similar special events to cater for them.
Thanks for commenting! :)