How old do you have t I be to go on a roller coaster by your self?

It depends on the amusement park and roller coaster. Some roller coasters have a minimum age of 42 inches, while others require passengers to be at least 48 inches tall. Some parks may also have other restrictions, such as a maximum weight limit or requiring riders to be accompanied by an adult. It's always best to check with the amusement park's website or a staff member for specific information about height requirements and other restrictions.