Complex sentence: From around the age of 15, he set his heart on becoming a great violin virtuoso, and he did become quite an accomplished player of the instrument, even publicly performing the last two movements of the Mendelssohn Violin Concerto in Helsinki.
Split into simpler sentences: From around the age of 15, he set his heart on becoming a great violinist. He became an accomplished player, even performing the last two movements of the Mendelssohn Violin Concerto in Helsinki.