Honestly It is a choice you make (USA)

Technically, the moment you make your first transaction you are already supposed to be a business. Obviously that is not the case though. The point in which you become an official business is when you decide that being one benefits you more than not being a business. For example. By becoming a LLC you have tax benefits and a corporate shield (Meaning no one can sue you but have to sue your business). Which can be helpful if you sell on an extremely large scale, (where you have little contact with the buyer) So honestly. It's all a matter of personal opinion.