The European Union Youth Orchestra began 50 years ago as a gleam in the eye of South African Lionel Bryer and his American wife, Joy. Their persuasive personalities convinced people in high places to make opportunities for talented young musicians from various cultures to perform together. From nine European countries, the membership has expanded to pre-professionals from 27 nations. This week, they perform at the Kennedy Center with Pinchas Zukerman, a heady experience for youngsters hailing from cities, villages and the countryside of the continent.