Complex sentence: Since becoming the youngest prizewinner at the International Tchaikovsky Competition in 1994, he proceeded to gain 2nd Prize at the Queen Elizabeth Competition in 1999, and in 2007 won First Prize at the Cleveland International Piano Competition.
Split into simpler sentences: Since becoming the youngest prizewinner at the International Tchaikovsky Competition in 1994, he proceeded to gain 2nd Prize at the Queen Elizabeth Competition in 1999. And in 2007 won First Prize at the Cleveland International Piano Competition.