The world may never see a better advertisement for 80 than Maya Plisetskaya. Gliding on to the stage at Moscow’s Kremlin Palace Theatre, at the end of a glittering gala on her actual birthday, the stunningly fresh-faced ballerina dances a lilting solo piece written for her by choreographer Roland Petit. And then, after a standing ovation from the 6,000-strong audience, she does it again.