As said, it depends on the wording of your insurance. As a general rule you can normally drive another car third party only with the owners permission provided the car is road legal and sometimes (I believe) the owner has to be insured in their own right as well. The wording of your policy may well say you must be over 25. Insurers cover themselves from every angle.
Not an answer to the question but in my opinion anyone who would let you drive an expensive car third party only would be daft I reckon. Also bear in mind that you would not bet covered yourself in the event of an accident, eg if you were seriously injured.