? Is it possible to start a Shopify store being under 18 years old and without any declared businesses

Yes, starting a Shopify business is not difficult, and in terms of an LLC and filling all the correct paper work to register a business you are of age for this.. however I would not start to do this process until you are generating sales.. startups can be slow.. I personally would wait until you hit your first 1k and then look into it, work on your product and marketing first.