[ Medicine ] Open Question : Should I take this medicine if im under age?

My cousin just came over to stay for a week, and just now she gets out this 'Curanail' treatment she was prescribed to use by her Doctor because her toe nails have a slight fungal infection (they have a yellow discoloration). She was prescribed it last month but only got down to using it tonight, while she was reading the instructions, it said DO NOT use if under the age of 18. My cousin is 15. My question is: can she still use this treatment for her toes even though shes underage? Does the fact that her Doctor prescribed it anyway mean she can take it? -Other Info- The treatment 'Curanail' is of Maximum Strength and is to be used once a week. Thanks :)