Complex sentence: The Sea Cadet Corps is for 10--18 year olds, it is a youth-based international organisation where youths can be part of a uniformed group which shows youths how to have sensible but serious fun.
Split into simpler sentences: The Sea Cadet Corps is for 10--18 year olds. It is a youth-based international organisation where youths can be part of a uniformed group which shows youths how to have sensible but serious fun.