Here is my 2 cents worth. I agree that age is a number and what is important is how mature both parties are. If the Dom is not mature enough in both or either life experience and emotional control this would be a real problem for a true sub that isn't looking to top from the bottom.

Youth has problems dealing with anger and such. A young Dom, I speak from my personal experience, may be arrogant and much more concerned with the power given him and less concerned with the sub's growth.

Control is something that has to be learned and that takes time.