A: Many businesses just need an ABN to get started. Some businesses need a licence, others don't. Best to check with your state business regulator, if you're not sure. To find out if you need to go down that track, best to have a chat with someone already running a business like this nearby.
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A: In Australia you can get started straight away, just contact the ATO and ask for your ABN and get started.
https://abr.gov.au/For-Business,-Super-f...
https://abr.gov.au/For-Business,-Super-f...
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.