But on Sunday, the nine-year-old will make his debut at the BBC Proms at the Royal Albert Hall in London.
He was born with perfect pitch, a talent shared by only one in 10,000 others, and he practises on the piano for eight hours a day. He made his concert debut, on the piano and cello, at six - the same age as Mozart was in 1762 when he gave his first performance.
At the Proms, Marc will perform a duet with Chinese pianist Lang Lang, playing Schubert's Fantasia in F minor.
Dressed in traditional white tie and tails, he finds it easier to lean against the piano stool rather than sit on it to perform.
Marc, from California, said: 'The problem is that my legs aren't straight. They bend a little, so I have to get very close to the piano and stretch my legs to reach.'
Marc started playing the piano at a friend's birthday party in Los Angeles when he was only two. As children sang Twinkle, Twinkle, Little Star, he toddled over to the piano and started playing the tune.
It astonished his Macau-born mother, Chloe, because that was the first time he'd been near a piano. He never looked back.
Six months later, Marc gave his first public recital, playing Beethoven. After his concert debut at six he won a 25,000 university fellowship the same year. Marc, an only-child, credits his 34-year-old mother - a single mum until recently - for nurturing his amazing musical talent because she played Beethoven CDs to him when he was in the womb.
He was recently featured in a National Geographic TV special in America called My Brilliant Brain, which explored where genius comes from.
At the Proms he will perform a piano duet with Lang Lang, a superstar in the classical music world who is as popular in the West as in his native China. Mrs Yu said home-schooling gives Marc the flexibility to travel and perform, while allowing him to learn at his own pace.
She said: 'What other children learn in eight hours a day he can squeeze into 30 minutes or an hour. This way, he can learn whatever he is interested in at that moment.' Now he also studies music composition part-time at the prestigious Colburn Music Conservatory in Los Angeles, and flies once a month to China for lessons at the Shanghai Conservatory of Music. - This is London
Marc said: 'I like playing music because it has a lot of different feelings - expressive, sad, excited and happy. I like playing difficult pieces, especially those that my teacher says no to.
'Practice makes perfect. You don't want a Beethoven piece to sound like something else. That's disrespectful to the composer.'
His mother insisted Marc is no bookworm. He likes playing in the park with other children his age, and recently developed a keen interest in card tricks - and gambling games like blackjack and poker.
Mrs Yu said: 'Sometimes his energy drives me crazy because he is running around or jumping up and down just seconds before he goes on stage. I would be like, "Can you please just meditate or calm down or shut up for two minutes?' But he can't.'
In his music, it's not just short legs that cause problems. Marc desperately wants to play Rachmaninov's second and third concertos, but they require a full eight-note spread across the piano keys and his hands are not yet big enough.
'It will be four or five years before his hands reach,' said his mother.
'No it won't,' said Marc. 'Things will be easier when I grow just a little bit more.' - This is London