You are too young to buy ANY car or truck. You have to be at least 18 to enter into a legal contract. And it doesn't matter how old the person is who will sign for you, when you pay cash for anything that's $10,000 or more, paperwork must be filled out and sent to the IRS. It's a federal law and no dealer will risk having his sales license revoked and his dealership shut down by not following it to the letter.
If the 30 year old who will sign for it doesn't have enough provable income to support such a purchase, you can be sure he will be thoroughly investigated.