A: In Australia, the Corporations Act 2001 does not have a minimum age you have to have reached before you can be a shareholder of a company, so 14YO is a real possibility. It even has the possibility of a single shareholder/director under particular circumstances. But you don't have to be a company to do most things.
You can run your own business just with an ABN number, as a single person contractor. Contact the ATO to get your ABN and just get started.
You can run a business with (an)other(s) with a partnership agreement and an ABN number. And you can run an investment with a Trust. Here are the different business types.
Once you have decided what is best for you, there are documents there to get you started.