Ballet didn’t take center stage for Boston Ballet principal Chyrstyn Fentroy until she was 18. Her late start, and the tenacity that helped her overcome it, would lead to an illustrious career with Dance Theatre of Harlem and Boston Ballet. With her crisp, clear-as-a-bell technique, radiant stage presence and spellbinding command of an audience, Fentroy’s uncertain beginning is difficult to imagine now.