The Missouri Department of Revenue told us that at 17 you could place the title of a vehicle in your name since there is not a minimum age limit to title a vehicle or register it.
However the legal age at which a person becomes competent to contract in Missouri is 18 years. Although a vehicle purchaser does not have to be at least 18 years old to own a vehicle, a contract to purchase a vehicle signed by anyone younger than 18 is voidable. The minor can release themselves from the sales contract, return the vehicle, and recover the vehicle purchase price paid, even though the vehicle has been used and deteriorated in value.
As a minor in MO it appears you can both buy a new or used car, as long as the person will do a contract with you since you are under 18 not all people will without also getting your parent or guardian to sign on with you, and title the vehicle in your name. Getting insurance may be an issue however since most insurers will require you to have a parent or guardian sign on the policy with you if you are under 18 years old.
In general one must be an adult to obtain an insurance policy on their own, so 18 years of age or older. If you are under 18, and thus still considered a minor, then normally insurance companies' guidelines will require you to have a parent or guardian sign on legal documents, such as an insurance policy, with you.
You can shop around here to see if we can help you with your young driver car insurance needs.