Is there any business an 18-year-old can start which can make him a millionaire in his early or mid 20s?
You start with building one successful business then you rinse and repeat. It may take owning 3 businesses similar to the first one to get to a million but you can’t have 2 until you start one that works.
Once you get one business off the ground you can expand it add new offerings, etc. but you still need to get one off the ground.