The World of Music: The Great Virtuosi (London: Gibbings, 1892), by Anna, Comptesse de Brémont (1864-1922), contains some information about Ernst not found in other sources, even if its fulsome style has a low information-to-word ratio. Although she was too young to have met Ernst, it seems probable she talked to people who had – and there were clearly many musicians and members of the British upper classes in the 1890s who recalled him clearly. The following passage confirms that Ernst was 16 when he undertook his first tour (in April 1829), and also that, prior to the tour, Böhm had been concerned at how hard his pupil worked. [My attention was drawn to this book by Jeremy Nicholas].
The serious manner in which young Ernst applied himself to his studies made a very favourable impression on his teachers, who made no secret of their opinion that he was destined to take rank among musicians. Böhm was especially struck by the intelligence and remarkable talent of his pupil, and manifested the keenest interest in his progress; but the unwearied pursuit of study, the unflagging energy, the ceaseless industry of the student, alarmed him, and he warned him against the consequences of overwork. Such admonitions, however, had little weight with Ernst, who felt already that the longest life was too short for the pursuit of art, which can only be understood and mastered by unremitting industry and patience, and ennobled by the dignified acquisition of knowledge. His zeal and application bore fruit readily. At sixteen years of age he had triumphed over the greatest difficulties of art, and he undertook a concert tour. pp.61-2