I suppose there will always be some kind of discrimination in choosing airline pilots for different reasons. Height and weight to a certain degree have practical issues i.e. a really fat person wouldn't be able to fit on the flight deck - look at the problems which arise when fat passengers are put next to thin ones. Also, the medical probably has something to say about weight being fairly proportional to height. The age thing is probably to do with potential length of service i.e. a younger person will have longer to serve than an older person.
It is horrible for people to be discriminated especially when it comes to looks, academic qualifications, or background. How does being an expert in partial fractions (A-Level Maths) make you a better pilot ? Surely it would be better to go by actual flying experience, but then it would create discrimination on financial grounds.
As for the David Beckham argument, he was probably in the right place at the right time. There are footballers out there who could have ate David Beckham for breakfast, but they were never discovered. Chances are, these footballers stopped playing football at a young age (maybe due to some trivial reason such as the games teacher didn't like them, or they were discriminated), and went to work in I.T !
Further to my last post, I'm not turning this board into a "I hate working in I.T" board, but it seems as if most people on here, who are discontent, and pursue training to become pilots come from the I.T background. Do we have any discontent accountants, lawyers, plumbers or tax collectors in here ?