Complex sentence: Mayer excelled in combined events from a young age and won the octathlon gold medal at the 2009 World Youth Championships and the decathlon gold medal at the 2010 World Junior Championships.
Split into simpler sentences: Mayer excelled in combined events from a young age. He won the octathlon gold medal at the 2009 World Youth Championships and the decathlon gold medal at the 2010 World Junior Championships.