There is no age limit per se, all you need is a license. The auto dealer has insurance to cover people that are test driving their cars, so that is not an issue.
If you are young however, a lot of dealers discriminate against younger buyers, seeing them as "test pilots". I STILL see this at age 24. A few months back, I went to a dealer asking to test drive a 2K Maxima SE. He told me "You can't afford this car." I have NEVER been more insulted in my life (seeing as I don't dress like a slob, and likely make more money than he does.), but there was nothing I could do about it.
Be prepared to be turned down, eventually you will find a dealer that will treat you with respect, and that is the one you want to deal with.