Just out of interest, are new jockeys introduced to the game over time and do jockeys retire/leave the game? I'm assuming there must be new apprentices introduced at some stage?

No. New jockeys aren't introduced. What you start with is what you will have for the length of your game. In previous versions of the game the jockeys had an age but it was just a number. It had no effect on gameplay. What was funny was on a really long saved game you'd be giving a leg up to your 120 year old jockey.

Thanks. So do apprentice jockeys always retain their status or are they just not replaced by new apprentices after they've exceeded their win allowance?

Great game but that's a bit naff really and definitely something for an update/next version in terms of adding realism. Likewise with trainers.

Apprentices lose the weight break after reaching certain win milestones. New ones aren't introduced. You can hire another when yours loses their weight break - if there is another apprentice left.